I'm coding an email for a project and man! it's such a pain. Every other client has it's own implementation and supported rules. Some don't allow even simple properties like background-image while some support most advanced rules like media queries

I've worked with people at companies where this was their only responsibility. Setting up emails for clients, making sure they pass a battery of tests and look great in all browsers and clients. It's an incredible PITA and it's not a set it and forget it thing. Clients can change month to month; spam filters change, etc...