Symptoms

• Users are no longer able to log in at all
• 'Bad Gateway' errors when accessing the Web application
• Occasional connection issues

Issue

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

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 maxchild value of the Cyrus section(see calculation above) :