Fixing default Plesk courier mail settings

By default a Courier IMAP configured server only allows 4 connections per IP which causes usually a lot of problems with nearly every eMail client, causing for example the following exception:

Unable to connect to your IMAP server. You may have exceeded the maximum number of connections to this server.

Then edit your imap config

sudo nano /etc/courier-imap/imapd

And extend the connection limits

...
MAXDAEMONS=80
...
MAXPERIP=40
...

Don't forget to restart the server

sudo /etc/init.d/courier-imap restart