Mulltiple mailbox connections fail on Plesk for Windows
Just selecting "allow multiple sessions" doesn't work, both on Server 2003 / Server 2008.
A registry tweak does do the trick. If you are on a Win system then follow the below steps:
Run: REGEDIT
Browse to: HKLM > Software > MailEnable > MailEnable > Services > POP
Then look for "concurrent access enabled" and make sure the setting is set to 1 (allow)
http://forum.parallels.com/showthread.php?t=107244