[de6u99er]

Your comment could explain why they need to have downtime for maintaining something. Maybe even a pressing security fix, that breaks things.

That being said, I don't understand why companies don't have test systems for validation of updade and maintenance procedures.

My systems actually always had two test systems. One which was identical with Prod, and another one where test-users could test new features coming out of development system(s).

[dathinab]

Just a guess, but I think they are currently in the process of slowly doing some major infrastructure migrations.

And doing such thinks without downtime can be hard, especially if you did some design mistake at the beginning which wasn't obvious and didn't cause problems until they scaled to a much larger customer base and/or provided additional services (storage, calendar) which originally whern't planed/considered when creating the mail service.