Suspicious message in /var/log/maillog on Plesk for Linux server: NOQUEUE: reject: RCPT from

Symptoms

On a Plesk for Linux server, the log file /var/log/maillog contains different errors NOQUEUE: reject: RCPT from:

• CONFIG_TEXT: plesk.example.com postfix/smtpd[18336]: NOQUEUE: reject: RCPT from unknown[203.0.113.2]: 554 5.7.1 <unknown[203.0.113.2]>: Client host rejected: Access denied; from=<user@domain.com> to=<user@example.com> proto=ESMTP helo=<domain.com>

• CONFIG_TEXT: plesk.example.com postfix/smtpd[26537]: NOQUEUE: reject: RCPT from some.unknown.domain.com[203.0.113.2]: 454 4.7.1 <info@example.com>: Relay access denied; from=<bot@bounce.mailing.somedomain.com> to=<info@example.com> proto=ESMTP helo=<some.unknown.domain.com>

• CONFIG_TEXT: plesk.example.com postfix/smtpd[29579]: NOQUEUE: reject: RCPT from unknown[203.0.113.2]: 550 5.1.1 <nouser@example.com>: Recipient address rejected: User unknown in virtual mailbox table; from=<user@example.com> to=<nouser@example.com> proto=ESMTP helo=<SOM-PC>

Cause

• The error Client host rejected: Access denied means that a client could not use allowed authentication methods. 
  

  Note: it could be caused by a misconfiguration:
  https://support.plesk.com/hc/en-us/articles/213364749

• The error Relay access denied means that a client tried to use a server as a relay, but the settings are too restrictive.
  

  Note: it could be caused by a misconfiguration. To change relay settings, use the option Tools & Settings > Mail Server Settings > Relay options.

• The error Recipient address rejected: User unknown in virtual mailbox table means that somebody tried to send mail to the nonexistent user.

Resolution

Most of the time no actions need to be performed since the mail works the way it is supposed to work.