From the Group Technical Director at the advertising agency I worked for (Barkley): "Man, if you guys had a way to test different email clients, you'd have the market cornered."
One of the biggest frustrations for the QA team there was trying to test e-mail campaigns across different devices and clients. Some support CSS a certain way, others don't. Some allow this and not that. With all the different mobile devices, apps, and platforms, it's REALLY difficult to code up an e-mail campaign and be sure it functions across the board. The email marketing industry is massive, so it would be an interesting space to dabble in.
This could be as straightforward as booting up different mail clients on our VMs and taking a screenshot to satisfy most needs, but an interactive or automated solution could kill it.