Mail replies do not use the receiving address as the reply from address
This may be because a certain account is locked into a particular SMTP server.


Open Mail Preferences by typing Command + , or Mail -> Preferences
Click the Accounts tab.
Click on an account and make sure "Account Information" is selected.
In the "Outgoing Mail Server" section, make sure the "Use only this server" is not checked.
Do this for every account.