Yggmail should report back to user if email failed to send

[rany2]

Is your feature request related to a problem? Please describe.
The yggmail service I'm sending the email to is down but I am not made aware.

Describe the solution you'd like
I would like for my yggmail server to return an email to me letting me know that sending that email failed and for what reason.

Describe alternatives you've considered
N/A

Additional context
Postfix does something like this by default.

[neilalexander]

Given that we’ll retry sending the mail pretty much indefinitely, that leaves us with two questions:

1. When should we notify that something went wrong, after the first try or successive ones?
2. For just how long should we attempt to re-send mails?
3. We might want to obey the Expires header in either case

[nxshock]

Looks like we needs new config option to set re-send attempts count. May be with additional re-send rate field...

[adbenitez]

normal email servers usually send an email from "mailer daemon" saying something like "after X hour your message couldn't be delivery to X@example.com, there is no action needed on your side"

[d47081]

As understand, not 'delivered' mails are pending in queue table (SQLite)

so if the lifetime of queue infinitive (until both nodes connected) it has no sense to make any alerts (that disturb just)