This has been a hot topic for several users, seach the forum at you should be able to find some detailed information, but last time I checked this requires you to recompile qmail.
Now here's the catch-22, qmail's license prevents someone from distributing a modified binary, so, despite the large demand for this feature, this is not something that's distributed in binary form and is a reason why it's not included with E-Smith/SME out of the box.
Hope this helped,
Nathan