Google, one of the main email service providers (ESPs)

Negative margins get removed by Gmail and others. So, no negative margins.

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...

iOS 9 and 10 support @supports. Make sure your ESP is not stripping it, or feel free to share your code so we can help you.