Matthew van Eerde wrote:
I would guess some combination of setting the envelope-from, From:, Sender:, and Reply-To: would generate optimal behavior.  The harder question is, what is optimal behavior?

On an interesting side note, I started a problem ticken on the MyMSN site, and every single email they send me is un-replyable.  I have to grab the email address by hand and put it back into the 'to' field.

So, whatever they are doing...