If you have a Plesk server where short mail names are enabled, upgrading to Plesk 8.4 can cause some issues. Valid logins may be rejected, and they’ll appear in your /usr/local/psa/var/log/maillog as “no such user”. You can correct the issue by switching to long mail names (click Server -> Mail in Plesk), or you can run a shell script provided by Parallels.

For further details, refer to the Plesk KB article “Mail users cannot get or send mail after upgrade to Plesk 8.4”