Deliverability Testing and Diagnostic Tools

Free, interactive tools for verifying a sending setup and diagnosing delivery problems. They fall into two families:

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

Use both: DNS-based tools confirm the records are right; message-based tools confirm the mail your infrastructure actually emits authenticates and aligns.

Quick reference

Tool	Input	Primary question it answers
aboutmy.email	Send an email to a generated address	Does this message authenticate, align, and comply with Gmail/Yahoo bulk-sender requirements? (30+ checks)
mail-tester.com	Send an email to a generated address	How does a stock SpamAssassin + auth/blocklist check score this message? (0–10)
learndmarc.com	Send an email to a generated address, or paste headers	Step-by-step: why does SPF/DKIM/DMARC pass or fail for this message?
internet.nl/test-mail	Domain name	Does this mail domain meet modern open standards (IPv6, DNSSEC, SPF/DKIM/DMARC, STARTTLS, DANE, RPKI)?
Google CheckMX	Domain (+ optional DKIM selector)	Are MX/SPF/DKIM/DMARC records sane, per Google's own resolver view?
MXToolbox Email Health	Domain or email address	Broad health sweep: 100+ blocklists, MX/SMTP/DNS tests, categorized problems/warnings/passed
check.spamhaus.org	IP, domain, or hash	Is this listed on a Spamhaus blocklist, why, and how do I get delisted?

Message-based tools

aboutmy.email (Word to the Wise)

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

What it checks (30+ checks):

• Authentication, in depth: not just SPF pass/fail but the actual DNS lookup performed, the records found, the IP checked against, and the result; likewise DKIM with the selector, public key, and signature verification; DMARC with alignment; BIMI if configured.
• Compliance: a dedicated section checking the message against the Gmail and Yahoo bulk-sender requirements (one-click unsubscribe/List-Unsubscribe, authentication, alignment, etc.).
• Headers and content: MIME structure analysis, header warnings.
• Images: every image in the message broken down by host, with file size, native geometry vs. rendered geometry, and warnings when large images are scaled down client-side.

Reading a report, in order: Authentication first (anything red there gets fixed before anything else), then Compliance (the Gmail/Yahoo requirements are not suggestions), then Headers/Content warnings, then the "Good Practice" checklist.

When to use: the single best pre-launch and post-change verification for a sending platform — new IP/domain/ESP setup, after changing DKIM keys or the From domain, or when validating Gmail/Yahoo sender-requirement compliance.

mail-tester.com

Made by Mailpoet/AcyMailing heritage folks; the classic "score out of 10" tester. The page generates a random address (xyz…@srv1.mail-tester.com-style); you send your message to it and click "Then check your score."

What it checks:

• SpamAssassin: a vanilla (out-of-the-box) SpamAssassin install scores the message; mail-tester reverses the sign so higher = better, targeting 10/10.
• Authentication: SPF, DKIM, DMARC evaluation of the received message.
• Blocklists: checks the sending IP/domain against major DNSBLs.
• Content quality: broken links, image/text ratio, HTML weight and validity issues, presence of an unsubscribe mechanism.

Free via the web interface with a daily cap on free checks (paid/API plans lift the limit). Results pages persist at their URL for a period, so they can be shared with a hosting provider or ESP.

Caveats: the score is a proxy — a stock SpamAssassin is far simpler than Gmail/Outlook filtering, and a 10/10 does not guarantee inbox placement (reputation and engagement dominate, see Foundations). Treat anything it flags as worth fixing, but don't chase the score as a goal.

When to use: quick sanity check of a template + infrastructure combination; convenient first triage when a customer reports spam-foldering.

learndmarc.com (URIports)

An educational/diagnostic visualizer sponsored by URIports. Send an email to the generated address at its server (neo.learndmarc.com) and it replays the validation pipeline in real time, step by step, on screen:

1. Connection: source IP, hostname (rDNS), envelope sender.
2. SPF: the lookup against the MAIL FROM domain, pass/fail, and SPF alignment with the From: domain.
3. DKIM: each signature found — selector, algorithm, verification result — and DKIM alignment.
4. DMARC: how the SPF/DKIM results and alignment combine into the final DMARC verdict under the domain's published policy.

Extras: a header analyzer (paste raw message source instead of sending), a 10-question quiz with explanations, and shareable/anonymizable/printable results. Best experienced on desktop.

When to use: the clearest tool for understanding and explaining an authentication failure — ideal when SPF passes but DMARC fails (alignment problems), when debugging forwarding breakage, or when teaching a customer what DMARC alignment means. It tests authentication only — no content, blocklist, or placement checks.

Domain/DNS-based tools

internet.nl mail test

Run by the Dutch Internet Standards Platform — "an initiative of the Internet community and the Dutch government." Input: a domain name of an email service. It tests adoption of modern open standards, several of which matter even for domains with no mail service (e.g. publishing null MX/SPF/DMARC on non-sending domains):

• IPv6 reachability of name servers and mail servers
• DNSSEC signing and validation of the domain and MX domains
• Authentication: DMARC, DKIM, SPF record presence and syntax/policy strictness (it flags lax policies like p=none or ~all)
• Transport security: STARTTLS configuration quality and DANE (TLSA records bound to the MX certificates)
• RPKI route-origin authorization for the mail server IPs

Output: an overall percentage score plus per-section and per-subtest results, with a shareable permalink (useful for handing to a hosting provider) and an embeddable widget.

Caveats: it measures standards adoption, not deliverability per se. DNSSEC, DANE, and RPKI are excellent hygiene and increasingly expected in Europe (and by some governments), but Gmail/Outlook placement does not require them. A modest score can coexist with perfect inboxing — read the section detail, not the headline number.

When to use: infrastructure audits, security/compliance reviews, and verifying STARTTLS/DANE after MX or certificate changes.

Google Admin Toolbox CheckMX

Google's free DNS checker at toolbox.googleapps.com/apps/checkmx/. Input: a domain name, plus an optional DKIM selector. Aimed at Google Workspace administrators but useful for any domain: it fetches and lints MX records, SPF, DKIM (for the given selector), and DMARC as Google's resolvers see them, and flags common DNS problems (missing/duplicate records, too many SPF DNS lookups, syntax errors, misconfigured priorities).

When to use: fast second opinion on DNS records — particularly valuable because it reflects Google's view of your records, the receiver that matters most for many senders. Good first check when Gmail specifically is rejecting or spam-foldering mail.

MXToolbox Email Health

Aggregated health report at mxtoolbox.com/emailhealth/. Input: a domain or email address. It fans out into "hundreds of domain/email/network performance tests," grouped into five sections (blacklist, mail server, web server, DNS, plus problems summary), each showing Errors / Warnings / Passed:

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

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

Caveats: with 100+ blocklists checked, expect noise — many listed blocklists have negligible real-world impact. Weigh a listing by the list's reach (Spamhaus ≫ obscure lists), as covered in Blocklists and Spamhaus.

When to use: broad first-pass triage on "our mail suddenly bounces everywhere" incidents, and periodic health sweeps of sending domains and MXes.

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

Spamhaus's official free checker — the only authoritative way to check and resolve Spamhaus listings (third-party lookups of Spamhaus zones can be stale or blocked). Input: an IP address, domain, or hash; it also auto-detects your client IP on page load and immediately warns if that IP is listed. No account is needed 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 it doesn't recur, and a guided removal request. CSS listings get a dedicated troubleshooting step requiring RFC 5321/5322 sending-practice compliance before removal. Removal is always free — paid "delisting services" are scams.

When to use: immediately upon any bounce mentioning Spamhaus (e.g. 554 … found in the Spamhaus blocklist. See https://check.spamhaus.org/), and proactively when commissioning new IPs (previous tenants' listings, PBL status of the range). Full zone/return-code detail: Blocklists and Spamhaus.

Where each tool fits in a deliverability workflow

Stage	Tools
Commissioning new IPs/domains	check.spamhaus.org (inherited listings, PBL status) → Google CheckMX + internet.nl (records/standards correct) → aboutmy.email (live message authenticates and aligns) — all before warm-up begins
Pre-launch of a campaign/template	mail-tester.com (content + stock-filter score), aboutmy.email (compliance + rendering weight)
Authentication change (new DKIM key, SPF edit, DMARC policy move)	learndmarc.com + aboutmy.email to verify pass and alignment; Google CheckMX for record syntax
Sudden bounces / block incident	Read the SMTP response first → check.spamhaus.org if Spamhaus is referenced → MXToolbox Email Health for a broad sweep → learndmarc.com if the rejection cites authentication
Periodic audit	MXToolbox Email Health (respecting the free 1/24h cap) + internet.nl; re-run aboutmy.email after any platform change

None of these tools measures actual inbox-vs-spam placement at real mailbox providers or your reputation there; for that, use receiver-side data — Google Postmaster Tools, Microsoft SNDS, and feedback loops — alongside your own engagement metrics (see Foundations).