SUMMARY: Arrgh! Harbor Freight, the biggest tool retailer in the US, had more than 300,000 AOL names on its email house list. Not one of these eager tool customers received the Harbor Freights' weekly alerts they'd requested. Why? AOL's filters decided they didn't like the HTML format the messages were in.
Marketer Martin Vrieze switched to text-only, and voila! his messages got through, and his sales went up by 20 percentage points within 30 days.
If you suspect your HTML isn't getting through (b-to-b emailers to the corporate world also have this problem) you should test text-only...