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