ISSUEYou have created a mailbox with an alias, enabled spam filtering and set it "Move to spam folder", but it is not working for the alias. Spam just goes to the Inbox folder.
THINGS TO TRYThis issue was discovered in Parallels Plesk Panel 10.x.
In order to fix this issue apply a hotfix on the server. Hotfix can be applied on Parallels Plesk Panel 10.x on 64bit Ubuntu10.x and 32 bit Ubuntu 8.x
Replace the file $PLESK_DIR/handlers/hook/spam with the appropriate file from the attachment.
Set up correct permissions on the file $PLESK_DIR/handlers/hook/spam after replacement.
# chmod 550 /usr/local/psa/handlers/hook/spam
Please see the following KB article to learn how to install Panel fixes in Linux: http://kb.sp.parallels.com/en/8083