With MacPorts

Besides mutt, also install vim which provides the vi command mutt defaults to.

In .muttrc,

# Body Format
set text_flowed
set send_charset=utf-8

and in .vimrc,

source $VIMRUNTIME/defaults.vim
 
augroup mail_flowed " {
	autocmd!
	autocmd FileType mail setlocal formatoptions+=w
	autocmd FileType mail setlocal colorcolumn=72
augroup END " }