Bruce suggested I read about OE-QuoteFix:
http://home.in.tum.de/~jain/software/oe-quotefix/

That information is helpful, but my main problem is not in the changes that
quoting causes. That just happened to occur in this series of messages.
Normally, I do not include entire old messages in my outgoing messages. In
fact, I do not permit it in an email group I run.

My problem is in my own original outgoing email messages. My new messages
that I send to others have greatly varying line lengths, no matter where
I've set my wrap length. It makes me crazy to have to edit the line breaks
repeatedly or else appalls me when I see how others receive my messages if I
don't take that time to edit over and over again.

Sarah