Forwarding Envelope & Message Headers – Exchange 2003 & Exchange 2007

Exchange 2007 uses a different Envelope FROM address (and Return-path) address when forwarding is enabled to an external email address.

Take the following situation. sender@example.net is an external sender, who has sent an email to receiver@example.org. receiver@example.org has a forward set up (using an AD contact) to receiver@example.com.

So to clarify, these are the addresses we are looking at:

So, for both Exchange 2003 and Exchange 2007, we are interested in the ‘forwarding’ part of this sequence of events (ie: the automated process when sending from original recipient to forwarding address).

The Envelope Header is what is used for routing the message. It is the MAIL FROM: and RCPT TO: of the SMTP conversation as set out in RFC 5321. When the forward process takes place, these will be the values used for each version of Exchange:

The key difference is that Exchange 2003 will use the ORIGINAL sender SMTP address, and not the domain for which it is authoritative. This has been changed in Exchange 2007, so that the MAIL FROM: header is set to the Original recipient, and the domain for which the email server IS authoritative.

Next is the message header information in the conversation:

The To and From of the message is what the MUA (Mail User Agent) or ‘email client’ will be presented with. They will not see a change really. They will know it is a forwarded email because it will appear to be addressed to the original receiver, and not the forwarding address. Note however, that the Return path address will be different  for each version of Exchange.

This information is worth noting when it comes to mail flow issues, or if you are attempting to whitelist for your forwarded mail.

Shaun