Postfix: temporarily suspend mail delivery (soft bounce)

When migrating our production RT instance to a new server recently, we needed to set up a "soft bounce" in Postfix -- essentially, to tell Postfix to keep incoming mails in the queue, but not deliver them.

My colleague came up with this (i.e., lines to go in @/etc/postfix/main.cf@):

## for rejecting all mails temporarily
transport_maps = static:retry
soft_bounce = yes