Unable to connect to mailbox on Plesk via IMAP/POP3: "Maximum number of connections from user+IP exceeded" OR "Maximum connection limit reached"

Symptoms

• On attempt to connect to a mailbox hosted on a Plesk for Linux server via IMAP/POP3, the following errors appear in the email client or webmail:

  ERROR:
  Unable to connect to your IMAP server.
  You may have exceeded the maximum number of connections to this server.

  ---

  Error reading from network: cause: connection closed by foreign host

  ---

  Cannot get mail
  Mail server mail.example.com is not responding

• The following error is found in the log file /var/log/maillog:

  dovecot: imap-login: Maximum number of connections from user+IP exceeded (mail_max_userip_connections=10)

  ---

  courier-imaps: Maximum connection limit reached for ::ffff: 203.0.113.2

Cause

By default, the IMAP/POP3 server Dovecot or Courier-IMAP that works on a Plesk server limits the number of inbound connections to prevent users from opening up too many concurrent sessions.
This restriction can impact legitimate users who have multiple computers connecting to a Dovecot/Courier-IMAP server from behind a firewall, or a single computer running a mail client that takes advantage of mailbox caching.

Resolution

1. Log in to Plesk.

2. Go to Tools & Settings > Mail Server Settings.

3. Increase the value of the option Maximum number of connections for a user per IP address and click OK at the bottom of the page to apply the change.