There have been multiple versions of the same basic concept -- when forwarding, replace the original return address with something that goes back to the forwarder; when the forwarder receives a bounce, it figures out, somehow, where the original message came from, and forwards the bounce too.
The possible implementations generally involve the same basic trade-off: if you do it the easy way, you essentially become an open relay that's exploitable using the bounce-return mechanism; or you can tighten up the
Agreed, a too simple solution will only create more problems.
way the bounces are processed, but then the whole thing turns into a complicated, fragile implementation that easily breaks for the slightest of reasons.
Complicated perhaps, but I still think it's necessary and worth the effort to get right.
The problem scenario I see is when user Joe changes ISP:s now and then. He doesn't want to give out a new email address to everyone he knows each time so he gets a more generic address at a forwarder.
If his ISP implements SPF checks then the forwarder must have some kind of rewriting scheme or mail will get rejected.
The problem here is that Joe cannot tell his ISP to turn of SPF checks for some servers (the forwarder's servers). He cannot control the SPF records of everyone who tries to mail him. The only thing he can control is the choice of forwarder. As it is right now courier is out of the question in such a position. Unless you get creative with dotcourier files. But that probably makes things even more complicated to get secure.
The reason I'm being so persistent is that I think courier is the best MTA for my needs and I'd hate to have to set up another server just to handle forwarding.
Rgds Pierre
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