If you recently upgraded to Plesk 8.4.0 with short names enabled, you may have found that it’s working with SMTP, but it doesn’t work with POP3 or IMAP. There’s a bug in the Plesk version that prevents the courier configuration from being updated.
To correct the issue, first make sure that Plesk has short names enabled (Server > Mail). Once you’ve confirmed that Plesk thinks it’s enabled, add
SHORTNAMES=1 to the following configuration files:
Restart courier-imap with
/etc/init.d/courier-imap restart and you should be all set.