The xdg-tools package provides a command xdg-email which opens a mail in a
compose window in the user's preferred email client.  It does not work like
sendmail; instead, it uses the command line for to, cc, and bcc addresses, the
subject, and the body.  (I've filed wishlist bugs on xdg-tools for a way to
accept the body on stdin to avoid command-line length limits, and for a way to
accept an mbox-formatted file.)  Git could supply a script (or an option to
git-send-email) that accepts formatted patches and calls xdg-email.  Like
git-imap-send, this would help users who do not run a local MTA.

- Josh Triplett