Invalid inbox.

[Buddy Brannan]

Yeah, sounds like (just from what you've said) that adding users
doesn't create mailboxes. This is easily fixed. Sorry, not running Red
Hat, but the procedure should be similar, excepting that the group
names might be different.

Do
touch /var/spool/mail/<mailboxname>

Then
chown <user> /var/spool/mail/<mailboxname> (probably both the same)
chgrp mail /var/spool/mail/*
chmod 660 /var/spool/mail/*

Naturally, you'll want to touch files for each of the mailboxes you
want (creating an empty file for mail to get saved into). 

Then try running your fetchmail.
-- 
Buddy Brannan, KB5ELV/3   | "And if the ground yawned,
Phone: (814) 455-7333     | I'd step to the side and say,
Email: davros at ycardz.com  | "Hey ground! I'm nobody's lunch!"
http://www.ycardz.com/    | --Eddie From Ohio