This may or may not be germane to your issue, but I had all kinds of trouble when I initially set up OS X server, migrating from - among other things - a separate mailserver. This related to using periods in usernames and also from creating second usernames (with periods) in WGM. There seems to be an issue with anything other than letters and numbers in usernames.
For around 20 years, our email addresses had been in the form firstname.lastname@domain, so my first run at setting up snow leopard server used these existing usernames. I am no longer certain of the details, but depending on the method used (server preferences or workgroup manager, as I recall), the system would either accept or reject usernames containing periods. Since I was concerned that using our legacy firstname.lastname usernames might be a problem, I checked with apple enterprise support, and the person I spoke to could not identify a reason why the periods might cause a problem, so I went ahead.
I cannot recall the specific initial problems, but there were several and I ended up purging my firstname.lastname users and going with usernames that had no periods or other offending characters. This time, in order to continue receiving mail at our legacy addresses, each user had a second "firstname.lastname" username entered in WGM. Among the problems I recall arising at this juncture was WGM becoming semi-disabled (I was unable to add users, for instance) and, in some cases, and for no apparent reason, the firstname.lastname usernames swapped places with the "legal" usernames in WGM.
I ended up purging the users and groups again, using only the "legal" usernames and no second username in WGM, and manually editing the etc/postfix/aliases file, to deal with our legacy firstname.lastname usernames. Everything has been well behaved for several weeks now, and adding a couple of second names that had no (apparently) illegal characters has not caused any problem.
