RFC5321 and multiple RCPT TO

Solution 1:

Different reasoning for the message itself - and notifications regarding that message.

The part where spelling out unmentioned recipients in the headers defeats the purpose refers to the message as it is transmitted to the recipients.

The notification about failed recipients goes back to the sender, who clearly at one point must have known all recipients addresses anyway.

• Telling the sender that some - even unmentioned in the message structure - recipients have not received the message? A-okay.
• Telling the recipients the full list of other recipients, after the addresses were deliberately kept out of the message structure? Not okay.