If your emails are landing in spam folders or bouncing before they reach the inbox, the problem almost always starts at the infrastructure level. MxToolbox is a purpose-built online diagnostic platform that helps teams run DNS checks, verify email authentication, perform blacklist monitoring, and track email deliverability in real time. Whether you manage a single domain or oversee complex email infrastructure for an entire enterprise, MxToolbox gives you the visibility needed to find and fix problems before they damage your sender reputation and kill campaign performance.
What Is MxToolbox and What Does It Do?
Quick Answer: MxToolbox is a free and paid online diagnostic platform that performs DNS lookups, MX record checks, SMTP diagnostics, blacklist monitoring, and email authentication verification. It helps IT administrators, marketers, and domain owners identify and resolve email deliverability issues quickly. The platform is available at mxtoolbox.com and serves millions of users globally as of 2026.
At its core, MxToolbox is a network diagnostics platform focused entirely on email infrastructure health. You enter a domain name or IP address, and the tool runs a comprehensive series of checks against live DNS data, public blacklists, and mail server configurations to return a detailed diagnostic report within seconds.
The platform sits at the intersection of IT operations and email marketing. System administrators use it to verify server configurations and troubleshoot delivery failures. Marketing teams rely on it to ensure campaigns actually reach the inbox rather than disappearing into junk folders or being silently rejected.
As of 2026, MxToolbox has evolved from a simple lookup utility into a full email deliverability monitoring suite, offering real-time alerts, a dedicated Delivery Center dashboard, and managed service options for enterprise teams. You can explore the complete tool suite directly at mxtoolbox.com.
Why Email Deliverability Matters More Than Ever in 2026
Email remains one of the highest-ROI digital channels available, but that return depends entirely on messages actually reaching recipients. Deliverability problems silently destroy campaign performance without any obvious error message appearing on the sender side.
According to Validity’s State of Email Report (2026), 21% of legitimate marketing emails never reach the inbox, going to spam folders or being blocked entirely. That is roughly one in five messages lost before a recipient ever has a chance to open or act on it.
According to Google’s Email Sender Guidelines (updated 2026), bulk senders must authenticate email with SPF, DKIM, and DMARC or face significantly higher rejection rates from Gmail, which processes billions of messages every single day. This policy shift made tools like MxToolbox essential rather than optional for serious senders.
According to data from Cisco Talos Intelligence (2026), over 85% of global email traffic is spam, which means inbox providers apply increasingly aggressive filtering. Even legitimate senders with clean lists can be caught in those filters if their DNS records are misconfigured or their IP appears on a blacklist.
Understanding your infrastructure status is no longer a technical nicety. It is a business-critical requirement for any organization that relies on email to drive revenue, communicate with customers, or deliver transactional messages reliably.
How Does MxToolbox Work? A Step-by-Step Overview
MxToolbox works by querying live public DNS records, querying blacklist databases in real time, and connecting to mail servers to test configuration. Here is exactly how the diagnostic process works from start to finish.
- Enter your domain or IP address into the MxToolbox SuperTool or a specific lookup tool such as the MX Lookup, Blacklist Check, or DNS Lookup.
- MxToolbox queries live DNS to retrieve your current MX records, A records, SPF records, DKIM keys, and DMARC policy in real time.
- The blacklist check queries over 100 public RBLs (Real-time Blackhole Lists) simultaneously to see if your sending IP or domain appears on any of them.
- SMTP diagnostics connect directly to your mail server to verify it is responding correctly, accepting connections, and reporting the correct banner and hostname.
- Email authentication checks validate SPF, DKIM, and DMARC records for syntax errors, alignment issues, and policy gaps that could cause authentication failures.
- The platform returns a scored diagnostic report with color-coded results showing what is passing, what has warnings, and what is critically failing.
- For paid users, Delivery Center monitors continuously and sends real-time alerts when a blacklisting event, DNS change, or authentication failure is detected.
What Tools Does MxToolbox Include?
MxToolbox is not a single tool. It is a suite of individual diagnostic utilities grouped together under one platform. Each tool targets a specific layer of email and DNS infrastructure.
MX Lookup Tool
The MX Lookup tool retrieves and displays all MX records associated with a domain. It shows the priority value for each mail server, which determines the order in which messages are delivered when multiple servers are configured. This is the starting point for diagnosing any email routing problem.
Blacklist Check Tool
The Blacklist Check tool queries over 100 DNS-based blacklists simultaneously and reports which ones have listed your IP or domain. A single blacklisting on a major RBL can cause significant delivery failures across Gmail, Outlook, and other major providers. This tool tells you exactly where you are listed so you can begin the delisting process.
DNS Lookup Tool
The DNS Lookup tool retrieves any type of DNS record for a domain including A, AAAA, CNAME, MX, TXT, NS, PTR, and SOA records. It queries live DNS servers rather than cached data, ensuring the results reflect your actual current configuration rather than outdated propagated values.
Email Health Check (Domain Health Report)
The Domain Health Check is the most comprehensive single-report tool in the MxToolbox suite. It runs over 30 individual diagnostic tests simultaneously covering DNS configuration, blacklist status, SMTP connectivity, SPF records, DKIM records, and DMARC policy. Results are scored and color-coded for fast triage.
SMTP Diagnostics
The SMTP diagnostic tool connects directly to your mail server and simulates the SMTP handshake to verify the server is responding correctly. It checks the server banner, connection response codes, and TLS availability. This is critical for identifying misconfigured mail servers that silently fail to accept or deliver messages.
SPF, DKIM, and DMARC Record Checks
MxToolbox provides dedicated lookup tools for each of the three core email authentication standards. The SPF check validates your SPF record syntax and checks for common errors like exceeding the 10 DNS lookup limit. The DKIM check verifies your public key is published and correctly formatted. The DMARC check reads your policy record and flags gaps such as a policy of none with no reporting addresses configured.
SuperTool
The SuperTool is MxToolbox’s universal diagnostic interface. You can prefix your query with a command type such as mx:, blacklist:, smtp:, or spf: to run any specific test from a single search bar. It is the fastest way to run targeted diagnostics without navigating individual tool pages.
MxToolbox Free vs. Paid Plans: Full Feature Comparison
MxToolbox offers a free tier with access to most core lookup tools and a set of paid subscription tiers designed for teams that need continuous monitoring, historical data, and automated alerts. Here is how the plans compare as of 2026.
| Feature | Free | Basic ($19/mo) | Business ($49/mo) | Enterprise (Custom) |
|---|---|---|---|---|
| MX, DNS, SMTP Lookups | Unlimited | Unlimited | Unlimited | Unlimited |
| Blacklist Check | Manual only | Manual only | Automated monitoring | Automated monitoring |
| Delivery Center Dashboard | No | Yes (limited) | Yes (full) | Yes (full) |
| Real-Time Alerts | No | Email alerts | Email + SMS alerts | Full custom alerting |
| Monitored Domains | 0 | 1 | Up to 10 | Unlimited |
| Historical Data Retention | No | 30 days | 90 days | Custom |
| API Access | No | No | Yes | Yes |
| Best For | One-time checks | Small businesses | Growing teams | Enterprise IT |
The free tier is sufficient for one-time diagnostic checks, troubleshooting specific issues, or verifying a DNS configuration change. Paid plans become necessary when you need proactive monitoring and want to know about blacklisting events immediately rather than discovering them after deliverability has already dropped.
How to Run a Blacklist Check with MxToolbox
Blacklist monitoring is one of the most-used features in MxToolbox. Running a check takes under 60 seconds and immediately tells you if any of over 100 major blacklists have flagged your sending infrastructure.
- Go to mxtoolbox.com/blacklists.aspx or use the SuperTool with the prefix blacklist:
- Enter the IP address of your outbound mail server or your sending domain in the input field.
- Click the Blacklist Check button to initiate the scan across all monitored RBLs.
- Review the results. Green rows indicate the IP or domain is not listed on that blacklist. Red rows indicate a live listing that requires action.
- For each red result, note the blacklist name and click through to the blacklist provider’s website to review their specific delisting procedure.
- Submit a delisting request directly with each blacklist provider. Most major lists process requests within 24 to 72 hours if the underlying issue has been resolved.
- Re-run the MxToolbox blacklist check after 48 hours to confirm the delisting has propagated across the RBL’s DNS infrastructure.
If you are blacklisted, the most important first step is identifying and fixing the root cause before requesting removal. Common causes include compromised accounts sending spam, open relay configurations, shared hosting IP reputation issues, or sudden spikes in bounce rates from purchased or stale email lists.
How to Check and Fix DNS Records Using MxToolbox
DNS errors are among the most common causes of email delivery failures, and they are often invisible to senders until major delivery problems appear. MxToolbox makes it straightforward to audit every layer of your DNS configuration.
Checking Your MX Records
Navigate to the MX Lookup tool and enter your domain. MxToolbox will return all MX records with their priority values, the resolved IP addresses of each mail server, and whether each server is responding correctly. A common issue is an MX record pointing to a CNAME rather than a direct A record, which violates RFC standards and can cause delivery failures with strict providers.
Validating Your SPF Record
Use the SPF Lookup tool to retrieve and parse your SPF record. MxToolbox automatically counts your DNS lookup chain and alerts you if you exceed the 10-lookup limit defined in RFC 7208. Exceeding this limit causes SPF to return a PermError, which many receiving servers interpret as authentication failure and use as a spam signal.
Verifying DKIM Keys
Enter your DKIM selector and domain into the DKIM Lookup tool. MxToolbox retrieves the public key from DNS and validates that it is correctly formatted and matches the expected key length. A missing or malformed DKIM record means every outbound message from your domain will fail DKIM verification at receiving servers.
Reading Your DMARC Policy
The DMARC Lookup tool fetches your DMARC TXT record from the _dmarc subdomain and parses each tag. It flags issues such as a policy set to none with no reporting addresses, which means you are getting no intelligence about authentication failures. It also identifies alignment mode settings that may be too strict or too permissive for your sending patterns.
MxToolbox Delivery Center: Continuous Monitoring Explained
The Delivery Center is MxToolbox’s proactive monitoring dashboard available to paid subscribers. Rather than running manual spot checks, Delivery Center continuously watches your sending infrastructure and alerts you the moment something changes.
Key capabilities of the Delivery Center include blacklist event notifications sent within minutes of a listing occurring, DNS change detection that alerts you when any DNS record for a monitored domain is modified, SMTP connectivity monitoring that detects when a mail server goes offline or stops accepting connections, and email authentication status tracking that watches for SPF, DKIM, or DMARC record changes.
According to MxToolbox’s own platform documentation, the Delivery Center checks monitored domains against blacklists multiple times per day rather than on-demand, ensuring that new listings are caught within hours rather than discovered days later when deliverability metrics have already declined significantly.
For teams managing email infrastructure at scale, the Delivery Center API allows integration with existing monitoring stacks, ticketing systems, and alerting pipelines, making MxToolbox a component of a broader infrastructure observability practice rather than a standalone point-in-time tool.
MxToolbox vs. Competing Email Diagnostic Tools
MxToolbox is not the only email diagnostic platform available, but it is the most comprehensive free option and one of the most trusted names in the space. Here is how it compares to the closest alternatives as of 2026.
| Tool | Primary Strength | Blacklist Check | DMARC Monitoring | Free Tier | Best For |
|---|---|---|---|---|---|
| MxToolbox | Full DNS and email diagnostics | 100+ RBLs | Yes (paid) | Yes (robust) | IT admins and marketers |
| Mail-Tester | Spam score testing | Limited | Basic check | Yes (limited) | Campaign pre-send testing |
| dmarcian | DMARC reporting and analysis | No | Advanced | Trial only | DMARC compliance teams |
| GlockApps | Inbox placement testing | Yes | Yes | Limited | Email marketers |
| Postmark DMARC | Free DMARC reporting | No | Yes (reports only) | Yes | Small teams starting with DMARC |
MxToolbox’s primary advantage is the breadth of its free diagnostic suite. For teams that only need occasional lookups, the free tier delivers more value than most paid alternatives. For teams needing continuous monitoring, the paid plans are competitively priced compared to enterprise-focused alternatives like dmarcian.
Who Should Use MxToolbox? Ideal Use Cases and User Types
MxToolbox serves a wide range of users across technical and non-technical roles. Understanding which use case applies to you helps you get the most out of the platform from day one.
- IT administrators and sysadmins use MxToolbox to verify DNS configuration after changes, troubleshoot mail server connectivity issues, and monitor infrastructure health for multiple domains.
- Email marketers and campaign managers use it to diagnose deliverability drops, check authentication record status before major sends, and confirm that ESPs are correctly aligned with their domain’s DNS settings.
- Domain owners and small business operators use it to run periodic health checks and catch blacklisting events early before customer communications are affected.
- Managed service providers (MSPs) use the Business and Enterprise tiers to monitor multiple client domains from a single dashboard and receive alerts when any client’s infrastructure status changes.
- Security teams use MxToolbox to verify that DMARC policies are enforced rather than just monitored, reducing the risk of domain spoofing and phishing attacks that use their organization’s domain.
- Developers and DevOps engineers use the MxToolbox API to integrate DNS and blacklist monitoring into automated deployment pipelines and infrastructure-as-code workflows.
Common Email Deliverability Problems MxToolbox Helps Diagnose
Understanding the most frequent root causes of deliverability failure helps you use MxToolbox more effectively as a diagnostic instrument rather than just a lookup utility.
Missing or Broken SPF Records
SPF records define which mail servers are authorized to send email on behalf of your domain. A missing SPF record means receiving servers have no way to verify whether your messages are legitimate, leading to higher spam filter scores. A broken SPF record with syntax errors or too many DNS lookups causes outright authentication failures.
DKIM Selector Not Found in DNS
When your email service provider or ESP rotates DKIM keys, the new public key must be published in your DNS before the old key is retired. A gap in this transition means every outbound message fails DKIM verification. MxToolbox’s DKIM Lookup tool detects this immediately and shows exactly what is and is not published in DNS for each selector.
DMARC Policy Set to None
A DMARC policy of none means you are monitoring authentication results but taking no action on failures. While this is appropriate during initial deployment, leaving it at none indefinitely means spoofed emails using your domain are not being rejected at receiving servers. MxToolbox flags this as a warning in the Domain Health Report.
Sending IP Listed on Major Blacklists
Blacklist listings are often unexpected. A shared hosting environment, a compromised user account, or a poorly segmented ESP IP pool can all result in your sending IP appearing on one or more RBLs without any action on your part. MxToolbox’s blacklist check identifies these listings within seconds.
Reverse DNS (PTR) Record Mismatch
Many receiving mail servers perform a reverse DNS lookup on the connecting IP address and expect it to resolve to a hostname that matches the SMTP greeting. A missing or mismatched PTR record is a common spam signal. MxToolbox’s SMTP diagnostic checks this automatically as part of the connection test.
How MxToolbox Supports Email Authentication Standards in 2026
The three pillars of modern email authentication are SPF, DKIM, and DMARC. MxToolbox provides dedicated tools for each and integrates all three into its Domain Health Report for a single-view assessment of your authentication posture.
SPF (Sender Policy Framework) is a DNS TXT record that lists all IP addresses and mail servers authorized to send email from your domain. MxToolbox validates SPF syntax, counts DNS lookups in the resolution chain, and identifies include mechanisms that add unnecessary complexity or push you over the lookup limit.
DKIM (DomainKeys Identified Mail) uses public-key cryptography to sign outbound messages. The sending server signs each message with a private key, and the receiving server retrieves the corresponding public key from DNS to verify the signature has not been altered in transit. MxToolbox verifies that the public key is correctly published and properly formatted for each selector you test.
DMARC (Domain-based Message Authentication, Reporting and Conformance) ties SPF and DKIM together with a policy that tells receiving servers what to do when authentication fails. Options are none (monitor only), quarantine (send to spam), or reject (block the message entirely). MxToolbox reads your DMARC record, identifies the current policy enforcement level, and flags any missing reporting addresses that would prevent you from receiving failure reports.
According to the M3AAWG Best Practices for Email Senders (2026 edition), all organizations sending more than 5,000 messages per day to Gmail or Yahoo addresses should have a DMARC policy of at minimum quarantine to meet inbox provider requirements and protect their domain from spoofing abuse.
Frequently Asked Questions
What is MxToolbox used for?
MxToolbox is used to diagnose and monitor email and DNS infrastructure. Its core uses include checking MX records, running blacklist scans across over 100 RBLs, verifying SPF, DKIM, and DMARC authentication records, testing SMTP server connectivity, and continuously monitoring domain health for IT teams and email marketers.
Is MxToolbox free to use?
Yes, MxToolbox offers a robust free tier that includes unlimited DNS lookups, MX record checks, blacklist checks, SMTP diagnostics, and email authentication verification. Paid plans starting at approximately 19 dollars per month add continuous monitoring, real-time alerts, historical data retention, and multi-domain support for teams with ongoing monitoring needs.
How do I check if my domain is blacklisted using MxToolbox?
Go to mxtoolbox.com/blacklists.aspx and enter your sending IP address or domain name. MxToolbox will query over 100 public blacklists simultaneously and display color-coded results showing which lists have flagged your IP or domain. Red results indicate active listings that require a delisting request with the specific blacklist provider.
What does an MX record check show?
An MX record check shows all mail exchanger records associated with a domain, including the hostname of each receiving mail server, its priority value, and the resolved IP address. It also tests whether each server is reachable and responding correctly. This tells you where inbound email for a domain is being routed and whether routing is working as expected.
How many blacklists does MxToolbox check?
MxToolbox checks over 100 DNS-based blacklists simultaneously in a single scan. These include major RBLs such as Spamhaus, Barracuda, SORBS, SpamCop, and many others that are used by inbox providers and mail servers worldwide to filter incoming messages based on the reputation of the sending IP address or domain.
What is the MxToolbox SuperTool?
The SuperTool is MxToolbox’s universal diagnostic interface that allows users to run any lookup type from a single search bar using command prefixes. For example, typing mx:yourdomain.com runs an MX lookup, while blacklist:192.168.1.1 runs a blacklist check on that IP. It is the fastest way to access any of MxToolbox’s diagnostic tools without navigating individual pages.
Can MxToolbox check DMARC records?
Yes. MxToolbox has a dedicated DMARC lookup tool that retrieves and parses your DMARC TXT record from DNS. It displays each DMARC tag and its value, identifies the current policy enforcement level, flags missing or misconfigured reporting addresses, and highlights alignment settings that may not match your intended authentication configuration.
What is the MxToolbox Delivery Center?
The Delivery Center is MxToolbox’s continuous monitoring dashboard available to paid subscribers. It monitors your sending infrastructure around the clock and sends real-time alerts when a blacklisting event occurs, a DNS record changes, an SMTP server goes offline, or an email authentication record is modified. It is designed for teams managing email infrastructure proactively rather than reactively.
How accurate is the MxToolbox blacklist check?
MxToolbox queries live DNS data from each blacklist provider in real time rather than relying on cached or aggregated databases. This means results reflect the actual current status of your IP or domain on each list at the time of the query. Results are as accurate as the individual blacklist’s own DNS infrastructure allows and are updated with each new scan.
Does MxToolbox have an API?
Yes. MxToolbox provides an API available to Business and Enterprise plan subscribers. The API allows developers and DevOps teams to programmatically run DNS lookups, blacklist checks, and SMTP diagnostics and integrate the results into monitoring dashboards, alerting pipelines, ticketing systems, or infrastructure-as-code workflows without visiting the web interface manually.
Why is my email going to spam even though I pass SPF and DKIM checks?
Passing SPF and DKIM alone does not guarantee inbox placement. Other factors including sending IP reputation, content quality, engagement history, list hygiene, and blacklist status all influence inbox placement decisions. MxToolbox’s Domain Health Report covers all of these layers and can identify non-authentication issues such as blacklist listings or DNS misconfigurations contributing to spam filtering.
How often should I run a domain health check with MxToolbox?
For active senders, running a manual domain health check at least once per month and before any major email campaign is a recommended baseline practice. Organizations sending high volumes daily should invest in a paid MxToolbox plan to enable continuous automated monitoring rather than relying on periodic manual checks that may miss blacklisting events between scans.
Final Thoughts: Is MxToolbox Worth Using in 2026?
MxToolbox remains the most comprehensive free email and DNS diagnostic platform available as of 2026. For anyone sending email at any volume, the free tools alone provide more diagnostic depth than most paid alternatives in their entry-tier plans.
The combination of DNS lookups, blacklist scanning across 100-plus RBLs, SMTP diagnostics, and email authentication verification in a single platform makes it an indispensable tool for IT administrators and email marketers alike. The paid Delivery Center tier adds the proactive monitoring layer that serious senders need to catch issues before they affect deliverability metrics.
If you are evaluating MxToolbox alongside other email deliverability and infrastructure tools, comparing features, pricing, and real user reviews is the smartest way to make an informed decision. Explore detailed comparisons, verified user reviews, and alternative platform options on SpotSaaS to find the email deliverability solution that best fits your team’s specific infrastructure needs and budget.
