No mail-user-agent defined for mu4e

[monsanto]

Currently we can't integrate mu4e with the standard Emacs mail functionality because we cannot define a mail-user-agent for mu4e. The main thing holding us back is that we need a compose function that has the same interface as compose-mail.

Perhaps the best thing to do would be to define a mu4e-compose-mail with the compose-mail interface, and have the current compose function (which just grabs the information using mu4e-hdrs-* calls) call mu4e-compose-mail.

[djcb]

I've added this;
(setq mail-user-agent 'mu4e-user-agent)
works now.