I have multiple accounts defined; about half enabled. One of the accounts which is not enabled is used only to send -- it actually belongs to my business partner and is rarely used. On occasion, I use it to send something for her. However, if I receive a message where we are both targeted by the remote sender, there is a corner case which will cause the other account to be used as the account I'm replying from. If the incoming message contains: Cc: main_address, send_only_address the reply works as expected. However if it contains: Cc: send_only_address, main_address then, the reply will select the send-only account for the reply[all]. I've verified this by reversing an incoming message's Cc: addresses. The bug is present no matter which folder the message resides in.
Due to unexpected behavior resulting from Compose/Writing Automatic account selection settings -- disable to resolve. Recommend adding a small note to this Preferences area.