But I have done about 10 setups for corporations from fresh and 5 micro****e exchange rip outs.
Also worked 2 years for a blue chip on contract.
It payed for me to become a pilot.
Yeah fine but .... none of it was on Raspberry Pi. And with the sort of background you purport to have, I'm even more surprised you consider it a remotely viable solution to providing email for 100 users.
Anyway... each to their own, but don't come back here crying looking for tea & sympathy (or just technical advice) when it all blows up in your face. If you're a full time ATPL then you've got even less time to waste babysitting some cheapo solution.
Good luck.... you'll need it.