When sending our html newsletter via email, should I include both versions? HTML and plain text, in other words, should it be a multipart email? If so, why?
Coding HTML email so that your core message can be read without images is the way to go. While most email clients block images by default, they can display HTML markups and CSS formatted layout and types just fine. And considering close to 90% of all email newsletter sent are HTML (sorry, can't find source now), the days of multi-part email is behind us.
This page provides great statistics on most popular email clients and their handling of images. (http://www.campaignmonitor.com/blog/post/3574/) You can also find great tips on creating compelling emails without relying on images at all.