Email(smtp) is not a bad choice for messaging in space(or anywhere really) it is a well understood robust protocol designed in a time when all networks were slow and intermittent. Exactly what you need in space.

IMAP probably not so much, It depends too much on having a good network. unless the imap server is on the spaceship(heh, spaceSHIP, that is an optimistic term, but it is all we have, so going with it), I would not expect it to work all that well.

I am not very familiar with outlooks game, Historically my beef with with it and thunderbird was their local data store, I mean it was not strictly speaking bad, but I was like "we have this great Maildir spec, why are you using this propriety database that is prone to corruption, even if you don't like Maildir million files approach at least use sqlite"