Postfix: bind 0.0.0.0 port 25: Address already in use Email / SMTP Origin: Network Last updated 4 Nov 2026

Postfix: fatal: bind 0.0.0.0 port 25: Address already in use

Postfix cannot start its SMTP listener because another process (often Sendmail, Exim or a second Postfix) already uses port 25.

Exact message you may see fatal: bind 0.0.0.0 port 25: Address already in usepostfix/master: fatal: bind 0.0.0.0 port 25: Address already in usepostfix master bind port 25 address already in useAddress already in use port 25cannot bind to port 25
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Seen on: SMTP Linux

Meaning

Only one program can listen on a port. Find the owner with ss -ltnp and stop or disable it. Common after installing Postfix on a system with Exim (Debian) or Sendmail (CentOS), or after a crashed master process left a stale lock.

Common causes

  • Exim, Sendmail or another MTA already running
  • A previous Postfix master process still alive
  • Docker container publishing port 25
  • Mail filter proxy bound to 25
  • Stale PID file

⚡ Quick fix

  1. sudo ss -ltnp | grep ':25 ' to find the process
  2. Disable the other MTA (systemctl disable --now exim4)
  3. Start Postfix again and check postfix check

Detailed fix by platform

Linux

  1. sudo ss -ltnp | grep ':25 ' and sudo systemctl disable --now exim4 sendmail; sudo systemctl restart postfix

How to diagnose

  1. Owner — Which process holds the port?
  2. Services — Another MTA enabled?
  3. Check — postfix check clean?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

421 4.7.0 Try again later (rate limited) — SMTP: 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. Mail sent from your IP address has been temporarily rate limited (Email / SMTP)421 4.7.0 Try again l…450 4.7.1 Greylisted, please try again later — SMTP: 450 4.7.1 <user@example.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/example.com.html (Email / SMTP)450 4.7.1 Greylisted,…451 4.3.0 Local error in processing — SMTP: 451 4.3.0 Error: queue file write error / 451 Requested action aborted: local error in processing (Email / SMTP)451 4.3.0 Local error…Outbound port 25 blocked — SMTP: telnet smtp.example.com 25 — Connection timed out (cloud provider blocks port 25) (Email / SMTP)Outbound port 25 bloc…Postfix: Host or domain name not found — Postfix: status=deferred (Host or domain name not found. Name service error for name=example.com type=A: Host not found, try again) (Email / SMTP)Postfix: Host or doma…Postfix: SASL authentication failed — Postfix: warning: SASL authentication failure: cannot connect to saslauthd server: No such file or directory (Email / SMTP)Postfix: SASL authent…Postfix: status=deferred (connect ... Connection timed out) — Postfix: status=deferred (connect to gmail-smtp-in.l.google.com[142.250.27.26]:25: Connection timed out) (Email / SMTP)Postfix: status=defer…Postfix: …

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