This was driving me crazy: I removed sendmail on a FC5 install and wanted to move over to postfix instead. I was trying to get Maildir format mailboxes working properly but it kept complaining about permissions:
Aug 25 13:51:02 xxx postfix/local: warning: maildir access problem for UID/GID=1001/1001: create /home/xxx/Maildir/tmp/1156528262.P32487.xxx: Permission denied
Aug 25 13:51:02 xxx postfix/local: warning: perhaps you need to create the maildirs in advance
neither procmail or postfix could write to $HOME in either mbox or maildir format. It was however devilering properly to /var/spool/mail in mbox format.
Then on a hunch I tried disabling SELinux... problem solved! I hope this can save others some time. SELinux has so far been nothing but a headache for me, it didn't play nice with PXE-booting either.