I have not confirmed this problem in any version other than Plesk 8.3.0, simply because I have not had the time.
According to the Parallels forums and documentation, you should be able to upgrade the version of Courier-IMAP running on the server to any later version.
I however attempted to upgrade to Courier-IMAP 4.7.0 and 4.8.0 using authlib 0.6.3 with no success.
Then I read the following entry in the Courier-IMAP Changelog:
2004-11-05 Mr. Sam
* pop3dserver.c (main): Authenticated address is in AUTHENTICATED,
not AUTHADDR, now.
It would appear that this one change, breaks the authpsa authentication module so that one can not implement later versions of Courier-IMAP on Plesk 8.3.0, since authpsa expects the authentication information in the AUTHADDR shell variable.