Automatically redeliver messages sent to holding pen when there's a single guess

[garethrees]

Extracted from #4584.

If there's a single guess, we can be reasonably confident that the message will go to the right place.

We'll want to be sure that this auto-delivery is recorded as an event.

[RichardTaylor]

We'll want to be sure that this auto-delivery is recorded as an event.

Perhaps an email to the team/support address could be generated, maybe for each event, or perhaps as a daily digest, reporting what had been done. Such an email could contain enough material to sense-check the re-delivery.

I suspect the "guesses" are sufficiently good, especially when there is only one, this wouldn't be needed though.

[garethrees]

Perhaps an email to the team/support address could be generated, maybe for each event, or perhaps as a daily digest, reporting what had been done.

Out of scope for the initial pass at this. Let's ticket that up if we start seeing problems reported from single-guess automatic delivery.

Can we add if the there's a header like X-Delivered-Suffix: -nnnnnn-xxxxxxxxx

That should be added to the "guessing" mechanism (#5988) so out of scope for this issue.