I did it in After Effects. Animation was not too difficult - the hard part is coming up with ideas
We use it to promote the station - DJs can send it as an ecard - either encouraging someone to listen or join up as a DJ.
I used to embed Flash in HTML email but nowadays it doesnt make it past spam filters, company security restrictions etc. Also, Outlook2007 has has a big impact on how HTML emails are created.
The best option is to have a static or animated gif, make it clear the pic is clickable and will take you to a vid then let people arrive at a dedicated landing page. Thats not without problems, but thats a separate discussion...