Feature: expose count/details of message deliveries

[ScottPeterJohnson]

(This issue was imported from Gitea) soma on September 5, 2021: Depending on setup (e.g., with greylisting), this may not be relevant.

If incoming e-mails can be dropped before delivery due to signals suggesting that the message is spam, it would be helpful to be able to look up the count of rejected messages addressed to one's e-mail addresses and summary details for the rejections (e.g., claimed sender address) and have some way of allowing the senders to retry.

(Daily delivery statistics by e-mail is a feature at another e-mail provider and was helpful for allowlisting in order to receive transactional e-mails from actual companies/institutions that somehow appeared questionable.)

Thank you in advance for considering this feature!