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...

Testing your designs is also crucial step during this process.

Remember, even the email clients with better HTML and CSS support have their own individual quirks and still require testing to see what’s possible.