Symptoms

Issue

When the maximum number of IMAP processes has been reached, users get login errors.

bm-ips may also no longer accept any connections.

Solution

You need to increase the maximum number of IMAP processes.
In general, you need to allow for 1.5 the number of users for a mixed webmail/thick client use. On installations where many users use thick clients, you need to allow for approximately 3 times the number of users.

  1. Count the number of processes used currently:

    pgrep -c imap
  2. Go to the administration console > System Configuration > Mail tab and increase the max child value in the Cyrus section (see calculation above):

    Note: the "Max child" default value is 200 (added when the field is left empty).