HTML Email
There are a great many specific and awful rules you need to follow to ensure a custom email renders well across the litany of ancient and inconsistent email clients out there. If you find yourself in a situation where you need to execute on an email design, do yourself a favor: leverage a tool or service to do it for you. Only as a last resort should you need to actually build an email completely from scratch.
Recommended tools/services:
TODO: tool/service list