No big deal, its pretty reliable, but also, but i got an MX at a friend's place. Never lost a mail, even if my server goes down for a couple of days.

Mail actually retries for a certain amount of time by default, so short downtime generally causes no problem at all. Longer ones do require the 2nd MX, but yeah, I do that. In about 15 years i think the backup MX helped twice so far (which is probably totally worth it)

the mail itself is also replicated there in fact