If your admin account is linked to email@example.com, and you send mail from firstname.lastname@example.org, it will show up as being posted by admin. If you have a WordPress user named “John Doe”, which is linked to email@example.com, make sure that you send emails from firstname.lastname@example.org. It doesn’t matter which email address Postie is checking. That is, if you send mail from email@example.com to firstname.lastname@example.org, 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.