I get quite a lot of emails from lists where the lines are suddenly cut off and continue on the next line. Now that kinda looks unprofessional. I've never thought of it, but it seems a good idea to try the full width approach to see if the response changes.

It's also an optical thing...using a fixed width makes the paragraphs look like paragraphs, which makes it somehow easier to read. When usaing full width a paragraph will mostly have 2 or maybe 3 lines.

I have to agree that with most emails you get from any list you can recognize from the first few lines it's a broadcast, but those who subscribe to a newsletter will know what to expect more or less, so I'm not sure that would make it any different.

Andre