- Dec 2020
The return (backward-pointing) address in the envelope is changed so that all error messages generated by the final deliveries will be returned to a list administrator, not to the message originator, who generally has no control over the contents of the list and will typically find error messages annoying.
- This the only difference between an SMTP alias and list; expanding recipients to RCPT commands is the same.
- Another great example to highlight the difference between SMTP envelope and mail data: the envelope's
MAIL FROM(see annotation) is different from to the mail data's rfc822
If the Klensin Standards Track [Page 19] RFC 5321 SMTP October 2008 recipient is known not to be a deliverable address, the SMTP server returns a 550 reply, typically with a string such as "no such user - " and the mailbox name (other circumstances and reply codes are possible).
Will the entire transaction be canceled? "Transaction" in the name of this a section implies yes, but what if all the recipients are valid?
See replies below but the answer is no; an
RCPTcommand is specified for each recipient in the mail data, these
RCPTcommands will be evaluated in sequence,, and any SMTP (error) notifications will be sent to the address in the
MAIL FROMcommand (which is not necessarily the same as the mail data's
There exist mailing lists that perform additional, sometimes extensive, modifications to a message and its envelope. Such mailing lists need to be viewed as full MUAs, which accept a delivery and post a new message.
I guess Mailman would qualify (e.g.,
The second step in the procedure is the RCPT command. This step of the procedure can be repeated any number of times.
This step of the procedure can be repeated any number of times. !
So the section
To expand an alias, the recipient mailer simply replaces the pseudo-mailbox address in the envelope with each of the expanded addresses in turn; the rest of the envelope and the message body are left unchanged. The message is then delivered or forwarded to each expanded address.
means that for each "to:" rfc822 message header in the mail data (i.e., SMTP envelope's DATA) a RCPT TO command will be added.
An important mail facility is a mechanism for multi-destination delivery of a single message, by transforming (or "expanding" or "exploding") a pseudo-mailbox address into a list of destination mailbox addresses.
So this section describes how distribution list addresses or mailing list addresses are expanded into individual email addresses of the members of those lists, and not how emails are delivered to multiple addresses when more than one recipient is specified in a MUA.