[Mimedefang] How to change envelope sender?

[kd6lvw at yahoo.com]

--- On Thu, 5/2/13, Benoit Panizzon <benoit.panizzon at imp.ch> wrote:
> Goal: Minimize Backscatter.
> 
> Problem: Customers do forward their email address to some other
> addresses which are have full quota or just any other problems.
> 
> Solution: Do this whole Aliasing and Forward-Address handling with
> MIMEDefang. As the local email address doing the forward is know
> to be good (or not, ....

I actually do this with sendmail's M4 rulesets and a pair of external maps, and I only rewrite the envelope sender field if SPF is detected in use for the sender domain.  Otherwise, such a rewrite is unnecessary.  The rewriting map is a small program that generates the new field value and stores the old and new values into the second map (which is a simple text map) for the possible event of a reply (i.e. a DSN from a subsequent server in the event it's forwarded again).  I expire entries after a week so that they can't be used to spam a sender at a later date.