How to use format=flowed with Neomutt and Vim

The format=flowed convention, specified in RFC 3676 is a way of formatting text-based messages so that they can be displayed nicely independently of the screen’s (or terminal’s) width. The core of the convention, roughly, is to distinguish soft line breaks, which the displaying software is free to re-arrange (and which are marked by a trailing space character), from hard line breaks, which are meant to always be honored.

See the above-mentioned RFC for more details on the format. The point of this short note is to explain how to configure the Neomutt mail client, along with the Vim editor, to use the format=flowed convention.

Neomutt configuration

Neomutt really needs only one configuration tweak in order to use format=flowed. Simply add the following line to the neomutt.rc configuration file:

set text_flowed

This will instruct Neomutt to add the format=flowed parameter to the Content-Type header of newly generated mails.

Vim configuration

Add the following line to the ~/.vim/filetype.vim file:

au BufNewFile,BufRead neomutt-* setf mail

This will lead Vim to use the mail filetype anytime it edits a file whose name starts with neomutt-, which will happen when the editor is fired from Neomutt to compose a message.

Then create a ~/.vim/after/ftplugin/mail.vim file, with the following contents:

setl tw=72
setl fo=awq
setl comments+=nb:>
match ErrorMsg '\s\+$'

The first line sets the maximal line length to 72 characters. The second enables automatic formatting of paragraphs, with a trailing white space indicating a paragraph continues in the next line. The third line specifies that lines starting with > are “comments” (so that quotes within a mail can be displayed differently from the rest of the message).

Finally, the last line allows to highlight the trailing space at the end of broken lines, to provide a visual distinction between “soft” and “hard” line breaks.