After some digging we managed to find out that Plesk restricts the number of IMAP servers and connections per IP address to pretty low numbers. To increase the values you need to change the following:
Find the file: /etc/courier-imap/imapd
Use SSH and then VI to open and edit the file: vi /etc/courier-imap/imapd
Change the following values: MAXDAEMONS=40 to MAXDAEMONS=80
Also change the following: MAXPERIP=4 to MAXPERIP=40
To save your file press Esc, then ZZ (in capitals), this shouldp ut your file to sleep and save the data.
Finally, restart courier-imap using: service courier-imap restart
Note: You should be able to edit the file using your favourite text editor via FTP however in our case the resultant file always crashed the imap service, which is why we ended up using VI.