Unable to login to email via IMAP: Connection to storage server failed: process_limit reached

Symptoms

• Unable to login to email via IMAP when Dovecot server is used with valid credentials:
  

  CONFIG_TEXT: Roundcube Connection to storage server failed

• Following errors pop-up in Roundcube webmail from time to time:
  

  PLESK_INFO: Empty startup greeting (localhost:143)

• In /var/log/maillog the following error message can be found:
  

  CONFIG_TEXT: dovecot: master: Warning : service(imap-login): process_limit (128) reached, client connections are being dropped
  dovecot: imap-login: Error: read(imap) failed: Remote closed connection (destination service { process_limit } reached?)

• If Watchdog extension is installed on the server the following error can be found in /var/log/plesk/modules/monit.log:
  

  CONFIG_TEXT: monit: embed_ssl_socket(): Openssl syscall error: Connection reset by peer!
  'dovecot' failed, cannot open a connection to INET[localhost:993]
  'dovecot' stop: /usr/local/psa/admin/bin/mailmng-service
  'dovecot' failed, cannot open a connection to INET[localhost:143]
  'dovecot' process is not running
  'dovecot' trying to restart 'dovecot' start: /usr/local/psa/admin/bin/mailmng-service

Cause

Dovecot connections limit exhausted: insufficient values for the "process_limit".

Resolution

1. Login to Plesk
2. Go to Tools & Settings > Mail Server Settings > change the value for Maximum number of connections (IMAP, POP3, IMAP over SSL/TLS, or POP3 over SSL/TLS) to a default one (1024) or higher: