Sender Rewriting Scheme

For a mail transfer agent (MTA), the Sender Rewriting Scheme (SRS) is a scheme for rewriting the envelope sender address of an email message, in view of remailing it. In this context, remailing is a kind of email forwarding. SRS was devised in order to forward email without breaking the Sender Policy Framework (SPF), in 2003.

In a number of cases, including change of email address and mailing lists, an MTA accepts an email message that is not destined to a local mailbox but needs to be forwarded. In such cases, the question arises of who deserves to receive any related bounce message. In general, that is either the author, or a person or other entity who administers the forwarding itself. Sending bounces to the author is administratively simpler and used to be accomplished by just keeping the original envelope sender. However, if the author address is subject to a strict SPF policy (-all) and the target MTA happens to enforce it, the forwarding transaction can be rejected. As a workaround, it is possible to synthesize on the fly a temporary bounce address that will direct any bounce back to the current MTA. The scheme provides for recovering the original envelope address, so that if a bounce does arrive, it can be forwarded along the reverse path—with an empty envelope sender this time.

While there are other workarounds, SRS is a fairly general one. It is interesting to note how its notion of reversing the path resembles the original routing dispositions for email, see below.

SRS is a form of variable envelope return path (VERP) inasmuch as it encodes the original envelope sender in the local part of the rewritten address. Consider example.com forwarding a message originally destined to bob@example.com to his new address bob@example.net:

The example above is adapted from Shevek. With respect to VERP, the local part (alice) is moved after her domain name (example.org), further adding a prefix (SRS0), a hash (HHH), and a timestamp (TT). That reflects an operational difference: Eventual bounces back to a VERP address are handled within the rewriting domain, and forged messages can at most unsubscribe some users, a kind of abuse that hasn't seen significant exploits in the last decades. Instead, SRS aims at remailing a possible bounce back to Alice, so that forged bounces can become an alluring technique for injecting spam apparently originating from the rewriting sender.

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