You can only setup .procmailrc files with "real" accounts, meaning if a user has got a shell/ftp account. So creating a .procmailrc file for a user who has only got a normal mail account is not possible.
Each mail account has it's own directory structure. Even the mXXXXXX mail accounts have a /home/mXXXXXX folder, but you can't access it because it's not your folder and the mail user can't login via shell.
For shell/ftp users the mail is individually stored in you /home/USER/Maildir folder.
As for as I know all the Spam is automagicly stored in the "Junk Mail" folder which is only accessible via webmail (that's why you get Junk Mail reports when mail is stores in that folder).
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