I have two email accounts being use in Thunderbird. Just recently I became aware that the outgoing server being used is what appears as the the from address and when someone replies it ONLY goes to that outgoing server email address. I changed the outgoing to the other server and the same thing happens. So the outgoing server determines the email address even if I enter a reply to address.

This is a recent problem. I have always, for many years, had multiple addresses managed by thunderbird and used one outgoing server but the emails were addressed from the specific email account being used.