I just upgraded to the 3.51 gpl version on a server.
This server also has mailman installed on it so in postfix I have two alias/virtual alias tables (one that postfix uses and one that only mailman modifys when mailing lists are added).
It appears that with this version virtualmin grabs the second table listed in the postfix config instead of the first (like it used to).
So the quick fix is to list the postfix tables second.
What happened to me is that any virtual server created since I upgraded had all of its aliases written to the mailman alias tables. When I started having weird mailing list problems I regenerated the alias files. When mailman does this it just overwrites them so I lost all my aliases added since the upgrade and mail started bouncing for those sites.