Edit: Security features described below have since been incorporated into mutt as the mailto_allow directive.
It’s time to configure your web browser to open mailto links using mutt. You know you want to.
You also know it’s going to be tricky, as mutt is a command-line program, and command line programs are like appliances that refuse to die. They work well but don’t match your cupboards from IKEA made from particle board. Still, mutt has a nice trick up its sleeve: It can process mailto arguments on the command line.
mutt mailto:mutt-users-request@mutt.org?Subject=Subscribe%20Mutt%20Users
There is only one problem: Mutt is unable to process every mailto attribute and fails on attributes it does not recognize (like Body).
mutt mailto:mutt-users-request@mutt.org?Subject=Subscribe%20Mutt%20Users&Body=subscribe
> Stopped
[The above mailto link is taken directly from the official mutt mailing list page. Mutt’s fleas have a good sense of irony.]
The solution is to wrap mutt in a script that parses mailto links. Using a whitelist of valid attributes has an added benefit of heightening security, as potentially malicious attributes are automatically discarded.
Use that script in a desktop file.
cat "$HOME/.local/share/applications/mutt-mailto.desktop"
> [Desktop Entry]
> Type=Application
> Name=Mutt Mailto Handler
> GenericName=MUA
> Comment=Supposedly sucks less.
> Exec=/home/ingvar/MuttMailto
> Terminal=true
> MimeType=x-scheme-handler/mailto
> NoDisplay=true
Inform the desktop environment.
update-desktop-database
Make it stick.
xdg-mime default mutt-mailto.desktop x-scheme-handler/mailto
Now you’re cooking with gas.
No Suck 
Mutt is not the problem. Your shell is intercepting the & in the mailto url to put mutt in the background, the trying to run `body=subscribe` as a command. If you quote the whole mailto url, the problem goes away.
… It all seems so obvious in retrospect. I suppose my brain failed to parse the ampersand on account of it not trailing. Thank you!
The article remains unedited, for purposes of humility (and whitelisting’s continued relevance to security).