SMTP Login Problem

[usdcservers]

I have just migrated from Ensim and cPanel to Plesk 8.1 and I am now getting complaints from users about not being able to log into SMTP with either password errors or the Server broke connection. Experimenting around if more then 4 users login at the sametime and a 5th tries to login the 5th one or higher will fail.

Coming from a Sendmail and Exim background I have no idea where to begin looking for a fix. Can anyone suggest a fix for this?

 

[usdcservers]

I have figured out what the problem was, maxperip was set to only 4 in imapd and pop3d.