I'm not going to argue postfix over sendmail or whatever as the default MTA, but it is a general consensus that UW-IMAP cannot handle high volumes. An updated package of cyrus-imap, or at the very least Courier should be included in RHL. Debian includes it. The cyrus-imap package was released with the last PowerTools release, but I notice that it isn't even included in your Advance Server lines.
*** Bug 73496 has been marked as a duplicate of this bug. ***
Please look to this as being a feature request for RHL Community and definately Enterprise. Having to spend 80 hours a week on this (while the guys downstairs just do an apt get) is not good for the pocketbook. [I just paid for several Enterprise contracts on money wasted.]
*** Bug 91647 has been marked as a duplicate of this bug. ***
cyrus is shipped in current releases.