Plesk: Upgrade to 8.4 causes “no such user” error in maillog
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”
Other posts I've written (some of them might even be good) 😉