Default sender in Apple Mail when replying and more than one account
Lets differentiate between NEW massages send from and Reply to sender.

There are 2 settings in Mail for each of the purpose.


One is for new mails:


That is the setting to either use a specific account or current Mail box account.


The other is for Replay-To.


The Reply-To settings are in each individual account settings, there you choose which account is to be used in Replay-To.


Preferences- Accounts - Outgoing mail Server (SMTP)
set the Server there and check the box use only this server.


Make sure you use (set) different server setting there for each account.