By default, the maximum number of concurrent connections from one ip in Qmail (plesk) is set to 4. Anymore than that and it will start to fail or reject the connections. Normally this isn’t an issue, but if you use an email client that tries to establish more than 4 connections at once, you will run into an issue.
To bypass this, you can edit the following files and change the MAXPERIP value to a different number. Remember, however the path’s may be different depending on your OS, so if it’s not the same as the path’s I list, you will need to locate/find or Grep for them. The path’s and files are;
/etc/courier-imap/imapd
/etc/courier-imap/pop3d
(another path is also /usr/lib/courier-imap/etc/imapd and /usr/lib/courier-imap/etc/pop3d)
Once you’ve made the edits, you will need to restart the mail service;
/etc/init.d/courier-imap stop and /etc/init.d/courier-imap start
=)
