If you are sending e-mail in
format=flowed (and you
should; for mutt, just add
.vimrc file for mail will look something like this:
setl tw=72 setl fo=watqc setl nojs setl nosmartindent
This makes the editor hard-wrap your text in a way that lets the e-mail client at the other side render it as it pleases (so, for instance, no more weirdly broken lines when someone reads your e-mail on a mobile device). The way this works is by leaving a trailing space where two lines can be merged together, and ending a line immediately after a non-blank character where a real hard break should occur.
However, after some e-mail editing we may leave some blanks at the end of lines that shouldn't be there, or there are some missing that should be included, if we are copying and pasting text, for instance. It would be good to visualize the trailing spaces for each line so we can make sure the format will be correct.
It turns out one only needs to add this line to our vim config:
" Mark trailing spaces, so we know we are doing flowed format right match ErrorMsg '\s\+$'