Postfix runs totally fine in a jail. There's just one or two things to keep in mind when setting it up.
A jail maps localhost addresses like 127.0.0.1 to the jail's address. So when you specify network blocks in access restrictions, filters and the like, make sure to use your jail IP rather than 127.0.0.1.
Connectiong to localhost works fine, but if a program checks to see where said connection is coming from it'll look like it's coming from your jail IP instead.