A couple questions and a couple comments.
First, does anyone know if Hotmail adds an "Envelope-to:" header line (in the extended headers) if that address does not appear in the To: header?
Reading the guide for Exim 4 (section 2.2.2), I'm reminded of three things: first, that things in (parentheses) are considered comments in headers; second, that the envelope could still be addressed to multiple spamgourmet users; third, that some MTAs will add an "Envelope-to:" header if the envelope address does not appear in the list of receivers (as in this case).
So ... a couple choices come to mind: the spamgourmet address could be put into parentheses in the To header and the true address added (not sure of implications on line length, reply emails, etc); spamgourmet could substitute the To: address(es?) and add either an Envelope-to: or custom header.
e.g. To:
hello.3.spamcowboy@spamgourmet.com becomes
To:
user@myisp.com (
hello.3.spamcowboy@spamgourmet.com)
Or add Envelope-to:
hello.3.spamcowboy@spamgourmet.com
Or add X-SG-to:
hello.3.spamcowboy@spamgourmet.com
Or perhaps this is a reasonable application for the Resent-xx headers (e.g. rfc2822, section a.3 (pp 43,44)). In which case, this might be good default behaviour.
Otherwise, of course, (as discussed) this may be better set as an option ...
As an aside (since I haven't looked at the code): if an email is sent to multiple spamgourmet receivers which resolve to the same 'true' domain, will spamgourmet handle the users together or independently?