It's probably straight-forward to do. Parse image links from the email, send those links to imgur, and replace the original image links with the imgur ones.
That's about 1% of the work. Email is old, arcane at times, and has a ton of interoperability and vendor-specific nonsense. Add in the fact that it _cannot_ go down for enterprise and that you have to do all of this at huge scale and it turns in to a pretty tough problem to solve.
I also used to work at message control. The architecture to support all of the legacy and new technology for email was very hard. Also architecting a system to ensure that it would never go down completely was a large challenge.
You also have to understand how providers deal with email to make sure delivery is never affected. We had to ensure 100% delivery and never fail.
