Comment 1 for bug 1255618

Firstly, the feedback to your related bug reports (bug 1255611 and bug 1255615) is relevant here.

Secondly, this bug report is correct in the sense that when sending out a multi-recipient mail_mail, the mail_mail.send() method does not keep track of which of the recipients have successfully been sent an email.

In the presence of an outgoing SMTP server that does not reliably accept outgoing messages, this means that all recipients are considered failed at once, with 2 consequences:
 - by default the recipients that have not yet received the email will never receive it unless a manual action is taken
 - if a manual retry of the failed delivery is attempted by the system administrator (via /Settings/Technical/Emails/Emails), all the recipients will be delivered again, possibly leading to duplicate messages *and* probably triggering the throttle limit again, making no further progress in the delivery.

So, in case of partial delivery failure the system could keep track of recipients who have already been delivered, and only retry the remaining ones. This does not solve the problem of handling the automated retry though.
However once we are able to partially retry a failed mail, we could imagine to distinguish permanent (5xx) and temporary (4xx) delivery failures, and automatically retry the temporary ones.

Note that this is considered a "nice-to-have" improvement rather than a bug, but mail delivery is sufficiently important to qualify it as a Low priority bug rather than a wishlist.

Thanks for your detailed bug report!