Email clients are known for handling HTML in a variety of broken ways, one worse than the other. From what I remember Notes is the worst one to handle, followed by Outlook. Since a few people use those clients, writing stuff they can display is required.

A funny thing I've read is that Outlook 2007 is apparently worse than the 2003 version. See e.g. http://www.google.se/search?q=outlook+html+rendering