That sounds oddly specific to be honest. Are you certain that the FROM header does contain the expected value or do you, by any chance, have a reply-to header set Zammad might react to? The from should be visible in the article in question if you click on its background. Zammad will then display Meta information (like the FROM).
I’m just asking, because I haven’t seen this kind of behavior unless the from is something else than the expactation.