Hey Bill,
Your account names have @ in them, correct? If so, the SASL version on FC5 does not accept them by default (we're fighting "Best Practices" when we use @ in the username...and so all of the software in the stack fights against us a little). You have to follow the instructions in the "usernames with @" FAQ found here:
http://www.virtualmin.com/faq/one-faq?faq_id=1511#51230The third paragraph is the one you're after. Basically, you just need to add "FLAGS=-r" to /etc/sysconfig/saslauthd.
We'll handle this automatically during installation soon...I just need to figure out which operating systems this applies to (the option doesn't exist on older versions of SASL, so we can't just modify everybodies configuration).