Ok, I may be missing some important point, but… can anyone explain why Exchange 2007 ignores the X-Sender and X-Receiver headers for Pickup folder submissions and why not for Replay folder submissions?

For those not familiar with the concept, IIS/Exchange supports submitting emails in a couple of ways. The primary way is SMTP, but emails may be submitted e.g. using the Store Driver or using the Pickup folder. This latter is a file system directory—put any legitimate email there and it will be picked up by IIS/Exchange for delivery. In pre-2007 versions, emails placed in the Pickup folder could have extra X-Sender and X-Receiver header fields to specify the envelope (P1) sender and the recipient list. When these headers were present, the MIME (P2) sender and recipients were ignored. This worked pretty much like SMTP, where the P1 sender/recipient could have been different than the P2 sender/recipient.

Exchange 2007 splits the previous Pickup folder functionality into a Pickup folder and a Replay folder. According to the documentation, the “Pickup directory is used by […] applications that must create and submit their own messages” and the Replay folder “receives messages from foreign gateway servers and resubmits messages that administrators export from the queues of Exchange 2007 servers”. Again, I may be missing the point, but the above just does not explain me why the Pickup folder lacks the X-Sender and X-Receiver support when the Replay folder has it. The ability to specify different P1 and P2 address information is a quite legitimate requirement by any software that creates new messages, but with the above limitation, the software developer has to abuse the Replay folder.