Deliverability Testing and Diagnostic Tools

When you set up a new sending configuration, or a delivery problem appears, free interactive tools can verify the setup and help diagnose the problem. They fall into two families:

• Message-based tools: you send a real email to a generated address, and the tool analyzes what actually arrived. Examples are aboutmy.email, mail-tester.com and learndmarc.com. These test the whole live path: the IP address that actually connected, the DKIM signature actually applied, and the headers your platform actually produces.
• Domain and DNS-based tools: you type a domain or an IP address, and the tool inspects published records and server behavior from the outside. Examples are internet.nl, Google CheckMX, MXToolbox Email Health and check.spamhaus.org. These test configuration, not a specific message.

Use both. DNS-based tools confirm that the records are right. Message-based tools confirm that the mail your infrastructure actually sends authenticates and aligns.

Quick reference

Message-based tools

aboutmy.email (Word to the Wise)

Built by Steve Atkins of Word to the Wise. Keep the page open (do not navigate away or refresh) and send an email to the one-time address it displays. Ideally, send a real campaign message from the real sending platform. You get a detailed report analyzing "authentication, size, structure and how well it complies with best practices." The tool is free.

What it checks (30+ checks):

• Authentication, in depth: not only whether SPF passed or failed, but the actual DNS lookup performed, the records found, the IP address checked, and the result. The same for DKIM, with the selector, public key and signature verification, and for DMARC, with alignment. BIMI is checked if configured.
• Compliance: a dedicated section that checks the message against the bulk-sender requirements of Gmail and Yahoo (one-click unsubscribe with List-Unsubscribe, authentication, alignment, etc.).
• Headers and content: analysis of the MIME structure, and warnings about headers.
• Images: every image in the message, listed by host, with its file size, its native and rendered dimensions, and warnings when large images are scaled down in the mail client.

Read a report in this order: Authentication first (fix anything red there before anything else), then Compliance (the requirements of Gmail and Yahoo are not suggestions), then the warnings about headers and content, then the "Good Practice" checklist.

When to use it: this is the best single check before launch and after a change to a sending platform. Use it for a new IP address, domain or ESP, after changing DKIM keys or the From domain, or to confirm compliance with the sender requirements of Gmail and Yahoo.

mail-tester.com

Made by people associated with Mailpoet and AcyMailing, this is the classic tester that gives a "score out of 10". The page generates a random address (xyz…@srv1.mail-tester.com-style). Send your message to it and click "Then check your score."

What it checks:

• SpamAssassin: an unmodified SpamAssassin install scores the message. mail-tester reverses the sign so that a higher score is better, with 10/10 as the target.
• Authentication: SPF, DKIM and DMARC evaluation of the received message.
• Blocklists: the sending IP address and domain, checked against major DNSBLs.
• Content quality: broken links, the ratio of images to text, HTML weight and validity issues, and whether an unsubscribe mechanism is present.

It is free through the web interface, with a daily limit on free checks. Paid and API plans lift the limit. Results pages stay available at their URL for a period, so you can share them with a hosting provider or ESP.

Caveats: the score is only an approximation. A standard SpamAssassin install is far simpler than the filtering at Gmail or Outlook, and a 10/10 does not guarantee inbox placement, because reputation and engagement dominate (see Foundations). Anything it flags is worth fixing, but do not chase the score as a goal.

When to use it: a quick check of a combination of template and infrastructure, and a convenient first step when a customer reports mail going to spam.

learndmarc.com (URIports)

A visual tool for learning and diagnosis, sponsored by URIports. Send an email to the generated address at its server (neo.learndmarc.com), and it replays the validation process on screen, step by step, in real time:

1. Connection: the source IP address, the hostname from reverse DNS, and the envelope sender.
2. SPF: the lookup against the MAIL FROM domain, whether it passed, and whether SPF aligns with the From: domain.
3. DKIM: each signature found, with its selector, algorithm and verification result, and whether DKIM aligns.
4. DMARC: how the SPF and DKIM results and their alignment combine into the final DMARC verdict under the domain's published policy.

It also offers a header analyzer (paste the raw message source instead of sending), a 10-question quiz with explanations, and results you can share, anonymize or print. It works best on a desktop.

When to use it: this is the clearest tool for understanding and explaining an authentication failure. It is ideal when SPF passes but DMARC fails (an alignment problem), when debugging failures caused by forwarding, or when teaching a customer what DMARC alignment means. It tests authentication only, with no checks of content, blocklists or placement.

Domain and DNS-based tools

internet.nl mail test

Run by the Dutch Internet Standards Platform, "an initiative of the Internet community and the Dutch government." You enter the domain name of an email service. It tests adoption of modern open standards. Several of them matter even for domains with no mail service, for example publishing a null MX record, SPF and DMARC on domains that do not send:

• IPv6 reachability of name servers and mail servers
• DNSSEC signing and validation of the domain and of the MX domains
• Authentication: whether DMARC, DKIM and SPF records exist, their syntax, and how strict the policy is (it flags lax policies such as p=none or ~all)
• Transport security: the quality of the STARTTLS configuration, and DANE (TLSA records bound to the MX certificates)
• RPKI route origin authorization for the IP addresses of the mail servers

The output is an overall percentage score, with results for each section and subtest, a permalink you can share (useful for sending to a hosting provider), and a widget you can embed.

Caveats: it measures adoption of standards, not deliverability as such. DNSSEC, DANE and RPKI are excellent hygiene and increasingly expected in Europe (and by some governments), but placement at Gmail or Outlook does not require them. A modest score can go with perfect inbox placement, so read the detail for each section, not the headline number.

When to use it: infrastructure audits, security and compliance reviews, and checking STARTTLS and DANE after a change of MX or certificate.

Google Admin Toolbox CheckMX

Google's free DNS checker, at toolbox.googleapps.com/apps/checkmx/. You enter a domain name, and optionally a DKIM selector. It is aimed at Google Workspace administrators but works for any domain. It fetches and checks the MX records, SPF, DKIM (for the given selector) and DMARC as Google's resolvers see them, and flags common DNS problems: missing or duplicate records, too many SPF DNS lookups, syntax errors, and misconfigured priorities.

When to use it: a fast second opinion on DNS records. It is particularly valuable because it shows how Google sees your records, and Google is the receiver that matters most for many senders. It is a good first check when Gmail in particular is rejecting mail or putting it in spam.

MXToolbox Email Health

An aggregated health report at mxtoolbox.com/emailhealth/. You enter a domain or an email address. It runs "hundreds of domain/email/network performance tests," grouped into five sections (blacklist, mail server, web server, DNS, and a summary of problems). Each section shows Errors, Warnings and Passed:

• Blocklists: the domain and the IP addresses of its mail servers, checked against 100+ DNSBLs (both IP and domain lists)
• Mail servers: each MX record probed for SMTP connectivity, banner and greeting checks, an open-relay test, a reverse DNS (PTR) match, TLS support, and transaction time
• DNS: 15+ tests against the authoritative name servers (SOA sanity, NS reachability, etc.), plus checks of the SPF and DMARC records

Free users get one Email Health Check every 24 hours. Paid tiers add unlimited checks, and a monitor that runs 30+ tests again every few minutes, with alerts. Individual MXToolbox lookups (a single blocklist check, spf:, dmarc:, smtp: etc.) remain free and unlimited for occasional use.

Caveats: with 100+ blocklists checked, expect noise, because many of the listed blocklists have almost no effect in practice. Weigh a listing by the reach of the list (Spamhaus matters far more than obscure lists), as explained in Blocklists and Spamhaus.

When to use it: a broad first check in incidents where "our mail suddenly bounces everywhere", and periodic health checks of sending domains and MX hosts.

check.spamhaus.org (IP & Domain Reputation Checker)

Spamhaus's official free checker, and the only authoritative way to check and resolve Spamhaus listings, because lookups of Spamhaus zones through third parties can be out of date or blocked. You enter an IP address, a domain or a hash. It also detects your own client IP address when the page loads, and warns you immediately if that address is listed. You do not need an account to check. The delisting workflow involves verification steps.

For a listed resource, it shows which dataset lists it (SBL, CSS, XBL, PBL, DBL…), why it was listed, what to fix so that the listing does not recur, and a guided removal request. CSS listings have a dedicated troubleshooting step that requires sending practices compliant with RFC 5321 and 5322 before removal. Removal is always free, and paid "delisting services" are scams.

When to use it: immediately after any bounce that mentions Spamhaus (for example 554 … found in the Spamhaus blocklist. See https://check.spamhaus.org/), and proactively when you bring new IP addresses into service (to find listings from previous users, and the PBL status of the range). For full detail on zones and return codes, see Blocklists and Spamhaus.

Where each tool fits in a deliverability workflow

None of these tools measures whether mail actually reaches the inbox or the spam folder at real mailbox providers, or your reputation there. For that, use data from the receivers, such as Google Postmaster Tools, Microsoft SNDS and feedback loops, together with your own engagement metrics (see Foundations).