If your admin account is linked to bar@gmail.com, and you send mail from bar@gmail.com, it will show up as being posted by admin. If you have a WordPress user named "John Doe", which is linked to johndoe@gmail.com, make sure that you send emails from johndoe@gmail.com. It doesn't matter which email address Postie is checking. That is, if you send mail from johndoe@gmail.com to foo@gmail.com, it gets posted as "John Doe". If you send an email to your Postie address from an email address that is not linked to a WordPress user, it will get posted as admin.