If you use the patched mmime to generate format=flowed emails, the
following code added to ~/.vimrc will give you a quick visual check that
the trailing whitespace is in the right place. Adjust the last two
autocommands if you configured a different maildir for drafts of outgoing
messages.

function! FixIndented()
	" remove spaces at end of indented lines
	silent! %s/^\s.*\zs \+$//
endfunction

function! TextFlowed()
	setl tw=72
	setl fo+=w
	setl nojs
	setl nosmartindent
	setl list
	set listchars=trail:•
endfunction

autocmd BufRead,BufNewFile */sent/cur/* call TextFlowed()
autocmd BufRead,BufWritePre */sent/cur/* call FixIndented()