set formatprg=fmt\ -w78
GNU fmt uses the line-breaking algorithm from TeX.
This works much, much better than vim’s built in formatter, and will make your emails look pretty; if like me you edit your emails in vim from mutt.
I can’t imagine using some lousy generic text box to edit email messages – although I am putting up with it for this blog! I should switch back to blosxom, or spend a few minutes hacking up my own RSS export gadget.
I could write emails without line-breaks, one line per paragraph, but then my emails will appear ridiculously wide in your average brain-dead mail reader. I’m looking at you, Outlook – do I want to read foot-and-a-half-wide columns of text? There’s a reason why newspapers keep their columns narrow – it’s easier to read!