Using the two mail accounts (one forwards to the other) works.
There is a fairly unique string for mail_userid in your prefs, but will not be able to test it for a few days.
..a filter would be nice as well though if you have time.
When you replicate your database from dev to production, it takes the values with it, which means that someone needs to remember to change it every time the db is sync’d.
Is there any other way to override the config ?