Reasonable questions all, but I don't have old Mercury logs - I only turned on that particular logging when it became an issue. I've checked Received headers in a current message and one received six months ago, and Mercury is declaring v4.74 in both.
It's possible that it's the level of problem that has changed. I've had the odd message before come to me, but I'm postmaster, so I just assumed an unknown problem and forwarded it. Now it's probably running at 20% or more of my wife's messages (haven't counted properly).
Another (possibly related) problem we have is spurious suppression of duplicate messages by Mercury. Since, in reality, we're both using the same mailbox on a given ISP, and the Envelope-to: headers are distinguishing which of us should get the mail, then if, say, we're both on the same mailing list, Mercury will suppress one copy of the message, which is infuriating, because you cannot easily work out what you've missed, or even which user missed it. At least if there were an option to turn duplicate suppression off, or even redirect duplicates for the postmaster's attention, there would be some control. I'm aware that the "Look only in these headers" setting should prevent this, but it does not appear to do so. I haven't even found a log where I can check what duplicates were suppressed. In at least one case, I'm having to use a rule in Pegasus Mail to autoforward the messages to the other account(s) and thus work around the issue, but I can only do that for mailing lists that I know are affected. I've also tried arranging for us to use different ISPs when registering on such lists, but not necessarily with great success. I'm not clear that I wouldn't even get spurious suppression sometimes if a friend included both of us in the addressees for the same message.
I'll try removing X-Envelope-to from the rules, because it seems to be irrelevant. As you say, it shouldn't make any difference, but who knows?
Happy to pass on logs and headers - should I PM you?
To state the obvious, I also have logs for messages that were delivered, including one where "duplicate" messages (from a Yahoo Group to which we both belong) were delivered properly to each of us, which is the result required. No real idea why it worked in that case and not in others.
Regards