At our (small) office we're deciding whether to have our MX record point to our local Exchange server. I was a bit worried about downtime, so thought that I ought to set up a backup MX server. Then I did a bit of research, and found a couple of posts that suggested it wasn't necessarily worth the effort, most notably because the sending MTA will queue the mails for a couple of days before giving up, which should be more than enough time to get your affairs in order.
I was all happy to go with that, but our local IT provider who we're buying our new server off insists that servers don't always queue the mails: sometimes they just immediately bounce. Can anyone comment on this position?
Also, I'm trying to find what the standard says on the matter (though I'm aware that real life and standards aren't always in alignment). The Wikipedia article on backup MX records says
The SMTP protocol establishes a store-and-forward network, and if a domain's mail servers are all offline, sending servers are required to queue messages destined for that domain to retry later.
Looking at RFC 821 I can't find that part, but I'm only scanning. Could anyone point out the relevant part if it exists? I'd also be interested in any part dealing with how long the mails ought to be kept for before discarding.