Common email delivery issues

Quick answer: Most delivery failures are readable: hard bounces (5xx codes like 550) mean permanent rejection — check the reason text; temporary deferrals (4xx) are often greylisting and cure themselves; silent spam-foldering means an authentication or reputation gap. The message headers and your mail log tell you which case you are in.

Overview

Diagnosing delivery starts from evidence, not guesswork: the bounce message, the Authentication-Results header of a received test mail, and the sending server's log. This article maps the common symptoms to causes and the fixes covered across this category.

Before you start

• A copy of the bounce (non-delivery report) or a test message that reached a mailbox you control.
• Access to the server's mail log (/var/log/mail.log on Debian/Ubuntu) and queue:

  postqueue -p

Step-by-step guide

1. Read the code: 550 5.7.x — policy/reputation rejection (check blacklists, PTR, SPF); 550 5.1.1 — recipient does not exist (remove the address); 452/421 4.x.x — temporary, often greylisting or rate limiting: the server retries automatically.
2. Inspect headers of a test message at Gmail/Outlook: Authentication-Results must show spf=pass, dkim=pass, dmarc=pass. Any fail points to the matching guide (DKIM, DMARC).
3. Check the queue and log for stuck or looping messages; growing queues usually mean DNS or connectivity problems from the server.
4. Greylisting: first contact deferred with a 4xx, delivered on retry minutes later — normal; ensure your MTA retries (all standard ones do).
5. Spam-foldering with all passes: reputation/engagement, not configuration — work the deliverability checklist.

Common issues

• Bounce says "listed at ...": the exact blacklist is named in the rejection — go straight to its lookup page.
• Delivery fine except one provider: that provider's internal reputation — register for its postmaster tools where offered.
• Mail vanishes with no bounce: usually delivered to spam, or swallowed by a local-delivery misconfiguration on the sending side.

When to contact support

If diagnostics point at the network path (port blocks, routing) rather than configuration, or your IP inherited a listing you cannot clear, open a ticket with the bounce text and timestamps.

Frequently asked questions

What is the difference between a 4xx and a 5xx bounce?

Codes starting with 4 are temporary — the sending server retries automatically, as with greylisting; codes starting with 5 are permanent rejections whose reason text names the fix.

Where do I see why my message was rejected?

In the bounce message text and in the sending server's mail log, such as /var/log/mail.log — the rejection line usually names the policy, blacklist or missing record involved.

Mail is delivered but lands in spam — what do I check?

Open the message headers: if SPF, DKIM and DMARC all show pass, the cause is reputation or engagement, so work through IP checks and gradual warm-up rather than configuration.

Related articles

• How to avoid email going to spam
• How to check IP reputation
• How to configure DKIM
• How to set reverse DNS / PTR record

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