Problem 

After installing FC5 and configuring the first in the LogWatch daily report gave me tis error (from the /var/log /security logfile):

--------------------- sasl auth daemon Begin ------------------------

**Unmatched Entries**

Deprecated pam_stack module called from service "imap"
Deprecated pam_stack module called from service "imap"

Solution

It appears that from FC5 onwards, lines like this in PAM configuration files:
auth required pam_stack.so service=system-auth
account required pam_stack.so service=system-auth

Should be replaced by:
auth include system-auth
account include system-auth

The error message will not be shown anymore, now a correct logentry will be added:
 runuser: pam_unix(runuser:session): session opened for user cyrus by (uid=0)