Click on More Settings …
At the General tab, change the Mail Account to your email address, e.g. johndoe@mydomain.com. Add “(POP3)” behind as shown above to indicate the account type if you want.
If you want your recipients to reply to a different address, e.g. johndoe@yahoo.com, fill it in the “Reply Email” blank. Otherwise, leave it empty so that it will default to your email address, i.e. johndoe@mydomain.com.