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A step-by-step diagnostic checklist for the two most common email problems: messages that don't send at all, and messages that send successfully but land in spam.

Problem 1: Email Not Sending

Step 1 — Check the Mail Service Is Running

sudo systemctl status postfix

Step 2 — Check the Mail Queue

mailq

A growing queue means mail is accepted but not being delivered — check the logs for the specific rejection reason.

Step 3 — Check the Mail Log

sudo tail -100 /var/log/mail.log

Look for the specific error next to the failed message — this almost always identifies the exact cause.

Step 4 — Test Port 25 Connectivity

telnet gmail-smtp-in.l.google.com 25

If this hangs or refuses, outbound port 25 may be blocked by your provider — a common cause of "stuck" mail, requiring the provider to unblock it for legitimate sending.

Step 5 — Verify DNS Resolution

dig MX gmail.com

If your server can't resolve recipient domains, mail delivery will fail entirely.

Problem 2: Email Sends but Goes to Spam

Step 1 — Verify SPF, DKIM, and DMARC All Pass

Send a test to Gmail, open the message, and select "Show original." All three should show PASS. If any fail, see How to Configure SPF, DKIM, and DMARC (Complete Guide).

Step 2 — Verify Reverse DNS

dig -x YOUR_SERVER_IP +short

Should return your mail hostname. If missing or incorrect, see How to Set Up Reverse DNS (PTR/rDNS) for Email Deliverability.

Step 3 — Check Blacklist Status

See How to Check and Remove Your IP from Email Blacklists (RBL).

Step 4 — Check IP Warm-Up Status

A brand-new, unwarmed IP sending significant volume is one of the most common causes of spam-folder placement — see IP Warm-Up: A Safe 30-Day Schedule for New Sending IPs.

Step 5 — Review Content for Spam Triggers

• Excessive capitalization or exclamation marks
• Spam-associated words in the subject line ("FREE", "ACT NOW", "GUARANTEED")
• Very high image-to-text ratio
• Suspicious or shortened links

Step 6 — Check List Quality

High bounce or complaint rates from a poor-quality list directly damage sender reputation — see Email Marketing Best Practices & Common Pitfalls to Avoid.

Full Diagnostic Checklist

1. systemctl status postfix — is the service running?
2. mailq and /var/log/mail.log — what's the exact error?
3. SPF/DKIM/DMARC — do all three pass in a Gmail header check?
4. Reverse DNS — does dig -x return the correct hostname?
5. Blacklist status — is the IP listed anywhere?
6. Warm-up status — is this IP new, and is volume appropriate for its age?
7. Content — any obvious spam triggers?

Common Errors and Their Meaning

Related Articles

• How to Configure SPF, DKIM, and DMARC (Complete Guide)
• How to Set Up Reverse DNS (PTR/rDNS) for Email Deliverability
• How to Check and Remove Your IP from Email Blacklists (RBL)
• How to Install and Configure Postfix as a Mail Transfer Agent