I've been developing websites for over a decade now, but quickly found that many of my habits in developing for the web are useless when developing for email clients. This has caused me an enormous amount of frustration, so I thought I would ask a question that would hopefully surface the best practices and necessary considerations for others like myself who may find themselves designing for gmail, outlook, etc. from time to time.

Example: <style>...</style> vs inline CSS.

In short: what transfers over from the web-world to the email-world, and what doesn't.