Hi!
What @waldrondigital posted essentially comes from
https://wiki.freepbx.org/display/PPS/Setup+Postfix+Manually
It's just people don't realize this is something they, at least currently, have to do that for office 365...
Microsoft servers are in their rights to to refuse to process emails with incorrect/unknown domains (or emall addresses which have not been given proper permissions, ie the "Send as")...
(As mail admins say, their servers, their rules...)
Anyway, for unknown domains if office 365 didn't block those they would most likely end up being blocked by the receiving servers.
(My MX for my domains and the ones I setup in the past certainly would...)
Honestly, as far as I am concerned, System Admin Pro should probably set up all these remappings by itself when you tell it you are relaying through office 365 servers (and maybe for other servers as well since it's not actually acceptable anywhere to send mails from unknown domains) and give it the permission to do it...
On a dedicated system it's not much of a problem for System Admin Pro to alter Postfix's configuration in such a way but if that system does more than FreePBX/Asterisk duty then I would be hesitant to have this done automatically, it might corrupt an otherwise working setup...
Have a nice day!
Nick