Fastmail had great developer tooling. Give yourself about two minutes from a issued command via slack or CLI and you'd have a brand new tagged developer box the live system that was a self contained infrastructure in a box of everything from the imap server to the user interface. Another command you could get as many copies for whatever development branch, etc.

The entire system replicated the real system (not user data but the software) and you could spin up test addresses and accounts and all sorts trivially. Beautiful!