What is the Sender Rewriting Scheme (SRS) in Email?
About
Sender Rewriting Scheme (SRS) is a process that rewrite the sender address:
[email protected]
into
[email protected]
It is mandatory in order to conform to the SPF scheme when the emails are forwarded.
With SRS, an MTA can circumvent SPF restrictions by replacing the envelope sender with a temporary email address from one of their own domains.
This temporary address is:
- bound to the original sender
- and only valid for a certain amount of time, which prevents abuse by spammers.
Example
If you get a forwarded email with SRS, you should see:
- the email rewrite
- the added Return Path
- a successful SPF test
Example from Gmail
Reference
https://www.libsrs2.org/srs/srs.pdf