Like One Recipient per Transaction. Defer for other Recipients, but delivers the messages promtly to the recipients, that want to accept the message, and remembers, the result of each action. When the SMTP client retries, the next recipient is accepted in the RCPT TO: phase, together with all recipients that share the same opinion on accepting a message with that recipient. At the end of DATA, the SMTP server communicates to the client the outcome of the iteration, but does not perform any delivery. Remember, the action was performed on the first iteration.
On the first iteration, the SMTP server remembers the Message-Id, the recipients and the actions per recipient.
The advantage of this approach is, that compared to no faked deferring, it does not cause delivery delay to the recipients.
The disadvantages are, that the sender might get a notification, that the delivery of her message was delayed, while in fact the message was delivered. Moreover, on retrying some SMTP clients change the Message-Id, so suppressing the action on a retry will not work and the receivers will get the same message more than once.
A disadvantage compared to the not-faked deferral is that during a later retry, the anti spam software can consider a mail as spam, while it was not considered as spam at the first try. To mitigate this disadvantage, the final mail store can perform anti-spam evaluation periodically after the delivery and mark messages as spam, even if they were not considered as spam at the time of delivery. However, such delayed reevaluation can be performed only until the user checks its inbox or spam mailbox, as subsequent moving of emails between the two mailboxes will confuse the users.