Postfix/PMG "451 4.3.0 Error: queue file write error"

If encounter periodically "queue file write error" running Postfix or Proxmox Mail Gateway, most likely due to not enough CPU/RAM/DiskIO resources on your system.

Workaround is try add below setting to /etc/postfix/main.cf.

smtpd_proxy_timeout = 240s #default settings is 100s

If it does not work then maybe is time to increase/upgrade your postfix/PMG system resources.

Out: 220 pmg.domain.com ESMTP Proxmox
In:  EHLO mail.superns.com
Out: 250-pmg.domain.com
Out: 250-PIPELINING
Out: 250-SIZE 157286400
Out: 250-VRFY
Out: 250-ETRN
Out: 250-ENHANCEDSTATUSCODES
Out: 250-8BITMIME
Out: 250-SMTPUTF8
Out: 250 CHUNKING
In:  MAIL FROM:<shaf@mexatraining20.ga> SIZE=37681
Out: 250 2.1.0 Ok
In:  RCPT TO:<user1@domain.com>
Out: 250 2.1.5 Ok
In:  DATA
Out: 354 End data with <CR><LF>.<CR><LF>
Out: 451 4.3.0 Error: queue file write error
In:  QUIT
Out: 221 2.0.0 Bye