If you are using GMX for SMTP relay. (Copiers...) as I detailed in other blog posts. I would recommend you log onto those accounts and send and receive a few email from the web mail client every once in awhile to keep the account active.
Even though I had a client using SMTP relay daily, I only once used the web mail client, and GMX closed the account for inactivity.
It was closed with zero warning and the account cannot be reactivated. So I will have to create another new GMX account for that same SMTP relay and reconfigure it all.