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 " }