In comparing my get.php file with the original from CE 1.8.1, it is completely different. I am afraid this was changed by an extension and if I write over it with the original that I will break something. Anybody have any experience with this? I searched online and my new version is actually InboX Mass Mailer. Thanks!
It's highly doubtful that an extension would change get.php. This file is used to retrieve media files stored in the database as BLOB objects, not for sending email.
It's more likely that due to extremely delayed installation of the SUPEE-5994 patch, someone has hacked this site to turn it into a spam relay.