Writing HTML for email is a pain. A lot of techniques that make writing HTML for websites don't work with certain email clients (I actually had a dream once that Gmail added support for CSS in the header...what a world that would be).
When I designed emails for America's Test Kitchen, it was like putting together a puzzle. When we wanted to include an ad where clicking different parts of it took you to different places, I had to design it in photoshop, slice it up, and reassemble the pieces in a table, and make sure everything rendered correctly across different email clients, which is a lot harder than it should be.
Every email I designed brought with it certain challenges and things that inexplicably wouldn't work, but I'm particularly proud of this one, because I had to work on top of an established template, while building in functionality we hadn't ever tried before.