Forwarding

Mail forwarding is the act of transferring a message from the server where the message recently arrived to another server that is deemed nearer to the intended recipient(s).

Meaning of the term in this site

We focus on two types of message transferring that servers do automatically upon receiving a message: possibly altering the envelope or leaving it intact.

Forwarding with a new envelope

The receiving server finds a forwarding recipe corresponding to one of the message's recipient addresses, and it honors it by

• replacing the recipient address with the ones listed in the recipe, and
• replacing the sender address, according to the recipe's policy.

The server address defines the behavior of the email system in the case of delivery failures: Even if the next server will accept the forwarded message, a delivery failure may occur further downstream. In that case, a failure notice should be delivered to the mailbox defined by setting the sender address at this stage. Therefore, there are a few considerations that may affect the policy:

1. Should the forwarding recipe be modified as a consequence of delivery failures?
2. Should the replaced recipient addresses be made known to the upstream sender?

This action is described in section 3.9.2 of RFC 5321, List. Note that the preceding section 3.9.1 of that RFC, Alias, describes a forwarding method useful for intra-domain forwarding. See below.

Forwarding with the same envelope

This is the operation that a backup MX accomplishes to deliver messages to primary MXes of the given domain, where further processing may occur. It may happen that a relay, e.g. the author's outgoing mail server, cannot reach a primary MX for a given recipient. That may occur if the target server is temporarily unreachable. Thus, secondary MXes can cope with network outages.

Forwarding agreement solution

Main article: forwarding agreement

An MX server who accepts such specific form of agreement should advertise it during the SMTP handshake. A forwarder may then choose to come into an agreement, or just submit mail without worrying about reputation and deliverability. Concluding the agreement requires a policy match involving the forwarder and the recipient. End users set their policies specifying relevant options at the target MX, or just accepting the default.

After the agreement has been set up, the forwarder is supplied with a valid user id and password which are only valid for forwarding that kind of mail to the given recipient.

Other acceptations of forwarding

That term is commonly used for the manual operation initiated by users by clicking on the Forward button of their MUAs. From the SMTP point of view, that action is not a forwarding, in the sense that the new message will have a new envelope, a new Message-ID, and a possibly altered content.

In addition, alias expansion is often referred by this term. The main difference between forwarding and alias expansion, according to SMTP, is that the latter does not alter the envelope sender. If the operation is carried out on the same server, it may not undergo a new SMTP transaction, and just write the message to a different location on disc. However, it is still quite common for servers to forward messages badly omitting to properly set a new envelope sender: while technically resembling an alias expansion, ill forwarding breaks protocols that depend on the envelope sender, e.g. SPF.

See also

• wp:Email forwarding
• Models for Mail Handling Systems
• RFC 5321 Simple Mail Transfer Protocol, section 3.9