In order to do an assessment of what email we lost in the outage (our website or relatively static, so email is the much bigger issue for us), I asked for and/or confirmed some information with the MDD staff. (Thanks @MikeDVB for the quick and personalized response). I thought I would post the info here in case it is helpful to others.
The first part of email loss would be messages from before the outage (i.e. messages that were actually received by my mail servers on MDD, but then lost in the outage
- Users who access mail via POP will have lost whatever mail they received between the last time they checked their mailbox and the start of the outage.
- Users who access their mail via webmail or IMAP will have lost whatever mail they received between MDD's backup of our account and the start of the outage
- For these messages, the sender would not have received a notice (i.e. bouncback) that the message had not been received.
The second part of the mail loss would be messages from during the outage (i.e. messages from when the server was offline)
- This would have been from the start of the outage to the restoration of the account.
- Senders would have received some sort of a notice that their message was not delivered.
There has been some comments in the forum (and I have seen the same in other discussions of mail delivery) that for messages sent to us during the outage, the sending server should queue the message and re-attempt delivery until the mail server comes back online (most sources suggest a 96-hour queue is typical).
However, Michael corrected this stating
If we had left the servers completely offline until we had the backups ready to restore - the messages would have stayed queued on the sending side of things.
But as we brought the actual servers back online within a couple of hours - and were just waiting on our backup systems to provide data to restore accounts the messages would likely have been bounced. The senders should have gotten bounces and known that their messages were not received.
So, it looks like we won't receive any emails sent during the outage (except for maybe the few-hour windows were the main servers were also offline) and that senders would have received a "permanent error" message like this https://forums.mddho...ge-17#entry7332