Sorry for the incoherent subject. I had a hard time trying to slim the problem down to a single sentence fragment. I'm new to Outlook but have ample experience with Excel VBA, so I'm just unfamiliar with what information I can and cannot get to in Outlook 2016 through VBA.

Here's a use case for what I'm after:
I have two accounts, A and B. They're both setup in Outlook so I can send from either.
I have A forwarding everything to B because I can get mail from B via push, which I prefer to fetch.
Someone sends a message to A, and I get the message in Outlook through account B.
When I reply to the message, I want Outlook to select A as the sending account, because the message was sent to A and it's natural for me to reply from A.

Is this something I can achieve with a macro set to a "reply button clicked" event? If so, I don't suppose someone in the community has already done this?

macOS Mail had this behavior (by default, I believe) and it worked great. In Outlook 2016 my default account is selected as the sending account for all replies regardless of the account to which the original message was actually addressed to.