Why test your email deliverability?
If your emails don't reach the inbox, you lose customers, sales, and trust. Without an email deliverability test, you're working blind.
An email tester (mail tester) analyzes your configuration by sending a real message from your production server. Unlike a simple DNS checker, it tests the complete email journey: authentication, routing, and standards compliance.
Common problems detected by email testing:
- Broken SPF → Your emails are blocked or marked as spam
- Invalid DKIM → Providers (Gmail, Outlook) don't trust your messages
- Missing DMARC → You're vulnerable to spoofing and phishing attacks
- Misconfigured DNS → MX, PTR or TXT records incomplete
Why do my emails go to spam?
"Why are my emails going to spam?" is the most common deliverability question, and the answer is almost always a missing signal that mailbox providers expect. The mail tester checks every one of them so you know exactly what to fix.
The usual causes:
- Failing authentication, no valid SPF, DKIM or DMARC, so the receiver cannot verify who really sent the message.
- Poor IP or domain reputation, a new IP, a shared IP burned by other senders, or a domain with past abuse.
- Blacklisting, your sending IP or domain is listed on an RBL like Spamhaus or Barracuda.
- Missing rDNS (PTR), your sending IP has no reverse DNS, an instant red flag for filters.
- Spammy content, misleading subject lines, broken HTML, link-only bodies or risky attachments.
It also depends on the mailbox provider:
- Gmail enforces its bulk-sender rules: valid SPF and DKIM, DMARC alignment and a one-click unsubscribe header are mandatory for high-volume senders, and a high spam-complaint rate sends you straight to spam.
- Outlook and Microsoft 365 weigh SmartScreen reputation heavily and are strict about clean rDNS and consistent sending IPs.
- Yahoo mirrors Gmail's authentication requirements and watches engagement and complaint rates closely.
Run a test below to see which of these signals is failing for your domain, then follow the recommendation for each flagged issue.
How to use the email tester in 3 steps
Step 1: Generate a test address
Click "Start email test" above. The system generates a unique address like:
test-abc123@mail-test.captaindns.com
This address is valid for 10 minutes and is only used for your test.
Step 2: Send from your mail server
Use your production server (not Gmail if you're testing your business domain) to send an email to this address.
Important for reliable testing:
- Send from the domain you want to test
- Use your actual production mail server (Mailchimp, SendGrid, Exchange, etc.)
- Subject and body don't matter (you can just send "test")
Step 3: View your test results
Wait 10-30 seconds. You get:
- ✅ Trust score out of 100 (deliverability score)
- ✅ SPF, DKIM, DMARC status (pass/fail with technical details)
- ✅ DNS issues detected (MX, PTR, missing records)
- ✅ Recommended actions to fix each problem
What does the email tester analyze exactly?
SPF (Sender Policy Framework)
Checks if your sending server is authorized to send for your domain. This is the first line of defense against spoofing.
The test verifies:
- SPF record exists and syntax is valid
- Your server's IP is authorized in the list
- No DNS lookup limit exceeded (10 max)
Common problems detected:
- ❌ Missing SPF → Add a TXT record to DNS
- ❌ Too many DNS lookups (>10) → Reduce
includedirectives - ❌ Unauthorized IP → Add your IP or include to SPF record
DKIM (DomainKeys Identified Mail)
Checks the cryptographic signature proving the email wasn't modified in transit.
The test verifies:
- DKIM signature is present in headers
- Public key exists in DNS (DKIM selector)
- Signature is valid, not expired, and matches the key
- Key strength is adequate (2048 bits minimum recommended)
Common problems detected:
- ❌ No DKIM signature → Enable DKIM on your mail server or provider
- ❌ Missing public key → Publish the DKIM record in DNS
- ❌ Invalid signature → Incompatible keys or email modified by a relay
DMARC (Domain-based Message Authentication, Reporting & Conformance)
Checks if your domain has a policy to handle SPF/DKIM failures. DMARC protects against phishing by telling recipients what to do with unauthenticated emails.
The test verifies:
- DMARC record exists (
_dmarc.captaindns.com) - A policy is defined (none/quarantine/reject)
- SPF and DKIM are aligned with the From domain
- Reporting addresses (rua/ruf) are configured
Common problems detected:
- ❌ Missing DMARC → Create the
_dmarc.captaindns.comrecord - ❌ Too permissive policy → Change from
p=nonetop=quarantineorp=reject - ❌ Alignment failed → Fix SPF or DKIM domain to match the From address
DNS configuration and infrastructure
The tester checks your overall email infrastructure configuration beyond authentication.
The test verifies:
- MX records correctly configured and accessible
- Reverse DNS (PTR) configured for sending IP
- Email headers compliant with standards (RFC 5322)
- No conflicting configuration between records
Real-world use cases
Case 1: Launching a new domain
Symptom: You're launching captaindns.com and want to send newsletters. You've never configured email authentication.
Action with the tester:
- Configure SPF, DKIM, DMARC in your DNS
- Run an email test from your server
- Fix issues flagged before sending to customers
Result: Avoid blacklisting from the first mass send. Target score: 90+.
Case 2: Emails suddenly landing in spam
Symptom: Your emails all land in spam since yesterday. Customers aren't receiving invoices or order confirmations.
Action with the tester:
- Test immediately with an email from your server
- Check the score and detailed diagnostics
- Identify the problem (often SPF or DKIM broken after a DNS change)
Result: Diagnosis in 30 seconds, delivery restored within hours.
Case 3: Email provider migration
Symptom: You're moving from Mailchimp to SendGrid (or any provider change). Need to verify everything works.
Action with the tester:
- Before migration: test and note your current score
- Configure new provider's SPF and DKIM in your DNS
- Test from the new provider
- Compare scores before/after
Result: No deliverability drop during migration. Immediate detection of configuration oversights.
Case 4: Regular production monitoring
Symptom: You send 100k emails/month. You want to maintain optimal deliverability and detect issues before they impact customers.
Action with the tester:
- Schedule weekly or monthly tests
- Monitor score trends
- Fix issues as soon as they appear
Result: Stable delivery rate at 95%+. No surprises with customers who "didn't receive" your emails.
Deliverability score interpretation
| Score | Meaning | Recommended action |
|---|---|---|
| 90-100 | Optimal | Configuration correct. Maintain monthly monitoring. |
| 70-89 | Good | Small improvements possible. Fix warnings. |
| 50-69 | Average | Authentication issues to fix quickly. |
| 0-49 | Critical | Emails likely blocked or in spam. Urgent action needed. |
Factors that impact the score:
- Valid and aligned SPF: +25 points
- Valid and aligned DKIM: +25 points
- DMARC configured with policy: +20 points
- Correct MX and PTR: +15 points
- Standards compliance: +15 points
FAQ - Frequently asked questions
Q: Why do my emails go to spam?
A: Emails land in spam because of missing or misconfigured authentication (SPF, DKIM, DMARC), poor sender or IP reputation, blacklisting, bad rDNS, or content that trips filters at Gmail, Outlook and Yahoo. The mail tester pinpoints which signal is failing so you can fix it and reach the inbox.
Q: How does the email tester work?
A: Click "Start email test", send an email to the generated address from your own mail server, and we automatically analyze SPF, DKIM, DMARC, headers, rDNS and DNS configuration. Results in 10-30 seconds, no signup.
Q: What does the deliverability score mean?
A: It's a spam score out of 100 that estimates your chances of reaching the inbox (inbox placement rate). It combines:
- Authentication: SPF, DKIM, DMARC pass or fail
- DNS configuration: MX, PTR, TXT records
- Compliance: RFC standards and best practices adherence
90-100 = optimal, 70-89 = good, 50-69 = issues to fix, below 50 = critical.
Q: Is the tester free?
A: Yes, 100% free with no signup. Generate a test address and send as many emails as you want. No usage limits.
Q: My score is low, what should I do?
A: The report shows exactly what's failing with fix instructions:
- SPF failed → Add your server to the SPF record
- DKIM missing → Enable DKIM at your email provider
- DMARC absent → Create a DMARC record
- DNS incorrect → Fix MX, PTR or rDNS records
Q: What is the difference between Mail Tester and the deliverability audit?
A: Mail Tester needs a real email to inspect the live message (SPF, DKIM and DMARC alignment, headers, content, sending IP), while the deliverability audit only reads your published DNS records without sending anything. Use the audit for a fast DNS review, and Mail Tester to validate real sending.
Q: What authentication protocols do you check?
A: SPF, DKIM and DMARC alignment, plus reverse DNS (rDNS/PTR), MX records and IP and domain blacklist status. We also review your headers for RFC compliance and detect conflicting DNS configuration.
Q: Can I test multiple domains or servers?
A: Yes, start a new test for each domain or server you want to verify. Each test generates a unique, independent address. You can test:
- Different sending domains
- Different providers (Mailchimp, SendGrid, etc.)
- Production vs staging servers
Q: Why do my emails land in spam on Gmail or Outlook specifically?
A: Gmail and Outlook enforce strict authentication for bulk senders, valid SPF and DKIM, DMARC alignment, clean rDNS and a low complaint rate. Yahoo has similar rules. Run the mail tester to see which signal fails for these providers, then fix the flagged record.
Complementary tools
| Tool | Purpose |
|---|---|
| Email deliverability audit | Check MX, SPF, DKIM and DMARC for a domain without sending an email |
| Email header analyzer | Analyze received email headers to diagnose issues |
| SPF record generator | Build a valid SPF record from your email providers |
| SPF inspector | Verify and fix your SPF record |
| SPF Flattener | Flatten your SPF to stay under the 10 DNS lookup limit |
| DKIM inspector | Validate your DKIM signature and key |
| DMARC inspector | Configure and test your DMARC policy |
| DMARC generator | Create a standards-compliant DMARC record |
| DNS propagation checker | Confirm your DNS records are propagated globally |
| IP Blacklist Checker | Check if your IP is blacklisted |
| Domain Blacklist Checker | Check if your domain is blacklisted |
| Phishing URL Checker | Check if a URL is flagged as phishing or malicious |
Useful resources
- Why your emails go to spam and how to fix it (CaptainDNS guide)
- DMARCbis: what's changing and how to prepare (CaptainDNS guide)
- DKIM2: what's new, removals and key dates (CaptainDNS guide)
- Google - Email sender guidelines (Gmail requirements for bulk senders)
- Microsoft - Email authentication in Microsoft 365 (Outlook/M365 guide)
- RFC 7208 - Sender Policy Framework (SPF) (official specification)
- RFC 6376 - DomainKeys Identified Mail (DKIM) (official specification)
- RFC 7489 - Domain-based Message Authentication (DMARC) (official specification)