In Mail.app I found that many of my emails are forced to be:
Content-Type: text/plain; charset=WINDOWS-1252; format=flowed; delsp=yes
charset changing depending on what characters are being used in the email. I am confounded because I have set the the message format to be 'Rich Text' in the preferences.
The reason for this is that I don't want my messages to be clipped at the 75 column mark (which many of the seem to be). At first I thought this was because I was using GMail as my SMTP server so I tried to force the output to be Rich Text. I prefer using plain text but want my email flowed since I do a lot of formatting text by hand, and sometimes line-breaks make the text less readable.
How do I force Mail to flow the text and not chop it? Here's a copy of the raw headers in my 'Sent' box. Mail displays this as flowed, but you can see that it's chopped in the raw source.