After upgrading to Plesk 7.5.4 no user can log into POP3 or IMAP mail accounts using a email client or Horde Webmail.

Please add the following lines at the end of /etc/courier-imap/imapd and
/etc/courier-imap/pop3d:

AUTHMODULES="authpsa"
MAILPASSWD="/usr/local/psa/qmail/users/poppasswd"

Then restart the courier-imap service.

Plesk