Emails are not delivered occasionally on Plesk server: warning: read TCP map reply from 127.0.0.1:12346: unexpected EOF (Connection timed out)

Symptoms

• Mail delivery does not work periodically. The following messages can be found in /var/log/maillog:

  hostname postfix/smtpd[2177845]: warning: read TCP map reply from 127.0.0.1:12346: unexpected EOF (Connection timed out)
  hostname postfix/smtpd[2177845]: warning: read TCP map reply from 127.0.0.1:12346: unexpected EOF (Connection timed out)

• These unexpected EOF errors itself are not an issue, they usually occur when smtp limits are reached, for example in /var/log/maillog:

  Jul 22 13:44:07 cp postfix / master [6027]: warning: service "smtp" (25) has reached its process limit "100": new clients may experience noticeable delays 2.

  OR

  Jul 22 18:16:40 cp postfix / smtpd [21470]: warning: Connection concurrency limit exceeded: 51 from unknown [203.0.113.2] for service smtp

Cause

SMTP connections exceed limits, causing SRS checker service to time out.

Resolution

• For service "smtp" (25) has reached its process limit "100" - check if default_process_limit needs to be increased or block malicious connections as described here: Postfix keeps stopping, preventing the sending of mail

• For "Connection concurrency limit exceeded: 51", check the limit for smtpd_client_connection_count_limit as described here: Email are not recieving on 587 submission port: Connection concurrency limit exceeded