Hey Jeremy,
Sorry for not responding, I missed these notifications for some reason.
The issue I faced ended up being caused by an image that was being used in my templates. It was a company logo being served up from AWS S3 and it had been working for some time. My guess is, somewhere along the way our email provider decided it did not like images from a domain that did not match the email domain. I switched the URL to an image on our website (same domain as our email) and it started working again.
Ronnie D.