Request for enhancement: Member “alias” addresses; paused addresses

I have a large list > 400 users, so naturally I have purchased the “Society” level to handle the number of users.
I have a user who represents a “strange” use-case that I need a way to handle.
He has two e-mails that are somehow recognized by gmail as equivalent.
He originally signed up as first.last@gmail.com and later somehow also added firstlast@gmail.com without the “.” …
Unfortunately he has multiple devices and is very inconsistent with how he sends e-mails.
He got blocked the first time he sent to the mailing group from firstlast@gmail.com because I had him listed as first.last@gmail.com.

So is there a way I can add both of his addresses and mark one as “paused” so that he can send from either, but will only receive mails on the active (not paused) address?

Alternatively, if there was a user property to capture alias addresses that could also work, but think of all the overhead every time you send an e-mail. Uggh.
It would be simple enough and also effective if I could just “pause” the secondary addresses. When sending you could let the database do the work with a select * from blah where active_send_address = true; kind of thing and a paused address would be active_send_address = false.

Any ideas how to solve this one without a code change?