[vidarh]

Usually they will retry for days. 4 days is conventional, and it's rare to see less than 24 hours... So you're right, it's a rare concern.

And of course if that's an issue, it doesn't take a huge amount of effort to run two - the secondary only needs to run the MTA with sufficiently longer retry intervals for passing mail on to the primary to give you whatever safety margin you'd like.

People seem to forget that e-mail started out in an environment where not all recipient hosts could even be expected to be online all the time - there's decades worth of solutions for doing store and forwards for hosts with intermittent connectivity, so dealing with failure of an MTA is easy.

My big issue with running my own mail server these days, though, isn't the inbound e-mail but deliverability of outbound e-mail. These days I'd be inclined to pass my outbound messages through somewhere like Sendgrid or Mandrill (at a cost) to let someone else deal with the deliverability problem.