I hate to write about problems I've had without working on solutions, but in the past I've had issues with simply "enabling" postfix without diving into the setup.
When I just added starting postfix with rc_update, I accumulated about 1 Gig of undelivered mail in the mail cache after a period of usage (I do make use of cron jobs). Obviously, the default configuration is insufficient, and I apologize for not having sorted the issue out. If anybody is feeling inspired, let us know how postfix is working for you now in the default configuration, and what we may need to implement to have it working 'ootb' if enabled without filling your fs with undelivered mail.
