Add callback for Mailgun

[inspiredearth]

This is an Enhancement suggestion.
This relates to use of Post SMTP with Mailgun, but might be just as applicable to other mail service provides/systems.

It would be great if the Post SMTP logs pulled through the actual result from Mailgrid. I've never had an issue with Post SMTP logging that a message failed to send, as it always makes it through to the Mailgrid SMTP server. But after that, it's not uncommon for Mailgrid to fail to deliver the message, typically due to their IP addresses being blacklisted.

Mailgrid offer a callback feature, to report the status of a message. Perhaps their API also has something similar (I don't know).

It would be great if Post SMTP made use of the Mailgrid callback, to provide true logging of the end result of each message that was sent... as in what eventually happened to the message. Was it delivered? Did it have a permanent fail, etc.