Correct use of SMTP "Sender" header?
- by Eric Rath
Our web application sends email messages to people when someone posts new content. Both sender and recipient have opted into receiving email messages from our application. When preparing such a message, we set the following SMTP headers:
FROM: author@example.com
TO: recipient@example.com
SENDER: webapp@mycompany.com
We chose to use the author's email address in the FROM header in an attempt to provide the best experience for the recipient; when they see the message in their mail client, the author is clear. To avoid the appearance of spoofing, we added the SENDER header (with our own company email address) to make it clear that we sent the message on the author's behalf. After reading RFCs 822 and 2822, this seems to be an intended use of the sender header.
Most receiving mail servers seem to handle this well; the email message is delivered normally (assuming the recipient mailbox exists, is not over quota, etc). However, when sending a message FROM an address in a domain TO an address in the same domain, some receiving domains reject the messages with a response like:
571 incorrect IP - psmtp (in reply to RCPT TO command)
I think this means the receiving server only saw that the FROM header address was in its own domain, and that the message originated from a server it didn't consider authorized to send messages for that domain. In other words, the receiving server ignored the SENDER header.
We have a workaround in place: the webapp keeps a list of such domains that seem to ignore the SENDER header, and when the FROM and TO headers are both in such a domain, it sets the FROM header to our own email address instead. But this list requires maintenance.
Is there a better way to achieve the desired experience? We'd like to be a "good citizen" of the net, and all parties involved -- senders and recipients -- want to participate and receive these messages. One alternative is to always use our company email address in the FROM header, and prepend the author's name/address to the subject, but this seems a little clumsy.
