Various operations in mail clients fail with error: Maximum number of connections from user+IP exceeded

Symptoms

• Some folders are missing in Outlook after mailbox synchronization;

• Checking mail in Thunderbird fails;

• mail_max_userip_connections parameter in the Dovecot configuration is set to 10:

  # dovecot -a | grep mail_max_userip_connectionsmail_max_userip_connections = 10

• Errors in /var/log/maillog:

  CONFIG_TEXT: Dec 21 09:51:18 example dovecot: imap-login: Maximum number of connections from user+IP exceeded (mail_max_userip_connections=10): user=user@example.com, method=PLAIN, rip=203.0.113.2, lip=203.0.113.1, TLS, session=<NfqTTyFE4QAqPdEQ>

Cause

The maximum number of connections for a user has been exceeded.

Resolution

Increase the maximum number of connections to avoid synchronization errors:

Tools & Settings > Mail Server Settings > Maximum number of connections for a user per IP address > increase the value and click OK.