[Mimedefang] How to change envelope sender?

[Benoit Panizzon]

Yo

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, but then we 
don't accept the email so won't create any backscatter) we know we can drop 
further errors happening while forwarding there.

In filter_recipient we check if the recipient is being forwarded to some other 
Destination. We can md_check_against_smtp_server to see if the recipient 
exists. But we don't know yet if the quota is full or similar. We only know 
this wen we queued and sent the actual email, but then we need to generate a 
bounce if this fails.
So let's change the sender to the address of the address of the local 
recipient who wants to have his emails forwarded somewhere else. So he get's 
the blame/bounce if his forward does not work.

Now I miss some kind of command to do that within MIMEDefang. $sender is read 
only I assume.

The complicated way would be to re-send / discard the email. Is there another 
solution?
This could also be used to implement propper SRS (which we actualy don't need 
anymore if we use the forwarder's address as sender).

Benoit Panizzon
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