Bill Cole wrote:
I don't see how this reduces the effort required on the receiving side in comparison to currently common practices.
Precisely - in fact, it increases the work the receiver has to do, probably substantially.
Consider: the offer/callback approach is identical to SMTP up to the DATA keyword.
The "offer" would have to have more-or-less the same information as the pre-DATA SMTP information in normal SMTP. A SMTP server can just as easily reject on that data pre-DATA, as to "choose not to do" the call back. So, up to this point, the offer/callback approach doesn't do any less work than normal SMTP.
Then offer/callback actually has to call back and retrieve the message. You also have to build in mechanisms to make sure that someone _else_ isn't doing the retrieval.
Then, if you do any DATA filtering, _both_ approaches have to do similar levels of work.
In other words, the offer/callback approach only causes you to expend more work actually implementing the callback _itself_, plus checking it for validity.