I've tried to locate some definitive documentation on Plesk 11 custom mail handlers, but failed quite miserably.
The documentation for Plesk 8.2 has some information, but is outdated of course:
http://download1.swsoft.com/Plesk/Plesk8.2/Doc/plesk-8.2-unix-mod-guide/38612.htm
http://download1.swsoft.com/Plesk/Plesk8.2/Doc/plesk-8.2-unix-mod-api/38849.htm
Sadly I can't find any documentation for the currently supported versions of Plesk (9-11). In fact, mailmng is not mentioned at all in the Plesk 11 Command Line Reference: http://download1.parallels.com/Plesk/PP11/11.0/Doc/en-US/online/plesk-unix-cli/ But the mailmng command is still available...
The documentation for Plesk 8.2 has some information, but is outdated of course:
http://download1.swsoft.com/Plesk/Plesk8.2/Doc/plesk-8.2-unix-mod-guide/38612.htm
http://download1.swsoft.com/Plesk/Plesk8.2/Doc/plesk-8.2-unix-mod-api/38849.htm
Sadly I can't find any documentation for the currently supported versions of Plesk (9-11). In fact, mailmng is not mentioned at all in the Plesk 11 Command Line Reference: http://download1.parallels.com/Plesk/PP11/11.0/Doc/en-US/online/plesk-unix-cli/ But the mailmng command is still available...