Default Email Author to Sales Channel

I have a client who set up their sales channels as a custom segment. They want any email correspondence, whether automated or manual, to be sent from a designated email address corresponding to the sales channel on the transaction.

I’ve added a field to the sales channel record to store the designated email. However, I can’t think of a way to ensure manual emails sent by employees use the sales channel address. I assume I could do this if they were leveraging subsidiaries. But they’re not using multiple subsidiaries. The sales channels are just segments.

Any suggestions?