From http://wiki.mutt.org/index.cgi?action=history&id=MuttFaq/Misc:
One simple way is:
# start a new eMail,
# fill in the template data & body,
# postpone the eMail,
# Copy the template eMail to your templates-folder (repeat for any other templates).
Then, to use a template, apply the resend-message
function on the desired template and continue to fill out the
missing data & body.You can also read a template from a file with partial headers and
body text when you launch mutt with -H .