Why is Mail switching the sending account on me?

When I post replies to Apple Developer List email-lists then sending account defaults to the account which I am registered to Apple-lists with. So it should be accepted. But half the time I get a rejection email from the relevant list and it is returned to a different account than the one I sent it from. Both are Gmail accounts and both are accounts I use in Mac Mail.

You'd think I'm sending from the wrong account but —apart from fact that a reply to a list email defaults to the correct account— this has happened countless times. I can't seem to see any correlation b/w sucessful send and anything that I'm doing differently.

Hair-pulling next.

Thanks.