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I've been seeing this warning in the last few months, when my email client is unable to connect to our dovecot IMAP server.

Warning: service(imap-login): process_limit reached, client connections are being dropped

Googling this I've seen suggestions to just up the limit (currently it's set to 144). Also a suggestion that some email clients using IMAP IDLE are holding open connections too long, so I should break the connection after 30 minutes.

Should I do one or both?

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Dovecot have two modes for login-services.

First is "secure mode". Each connection processed by separate instance of pop/imap-login that is persistent until client disconnects. There is limit for maximum concurrent connection and if it is exhausted new connections will be refused because of process_limit reached.

Second is the "performance mode". All connections processed by single instance that hasn't limits. Have to say that performance mode not so insecure as you can suppose, but rather "secure mode" is "supersecure".

If you got the message you can increase process_limit or switch login to performance mode:

service imap-login {
  inet_listener imap {
    port        = 143
  }
  service_count = 0 # performance mode. set to 1 for secure mode
  process_min_avail = 1
}
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    Dovecot has more than a dozen configuration files, so it would improve your answer if you stated in which configuration file this edit is made. I found (by searching them) it is the 10-master.conf file.
    – Martin Marshall
    Oct 22, 2018 at 16:31
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    @MartinMarshall Splitted config isn't mandatory and intended for easier integration with GUI configurators. doveconf -a shows the whole configuration as a single list of options. Version 1.x of dovecot used the monolitic config file by default and this is preferred format as long as we edit it by hands.
    – Kondybas
    Oct 22, 2018 at 23:27
  • Thank you so much! Often with nextcloud + rainloop the message list could not be loaded after a long delay when switching between folders and messages too fast. Very annoying when you try to forward 20 emails one by one every month. With this setting (server_count=0 and process_min_avail=1 under service imap-login) the problem is completely solved!
    – Pete
    Aug 21, 2022 at 22:44
  • Detailed explanations are here: doc.dovecot.org/admin_manual/login_processes. To find all your process_limit settings, which may be in different config files : grep -r process_limit /etc/dovecot/. The easiest is probably to just increase the default_process_limit.
    – mivk
    Jan 10, 2023 at 12:56

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