How / where is `C-c C-j` bound? In my Emacs, that combo isn't
bound at all. If it's a global binding that you configured
yourself, you might consider using `C-C j` instead, which is a
combo that no major or minor mode is supposed to override, since
`C-c <letter>` bindings are reserved for the user. (That's why
e.g., the org-mode manual suggests setting `C-c a`, `C-c l` and
`C-c c`, instead of org-mode setting them itself).
If it's a global setting configured by some package, it should be
enough to bind `C-c C-j` to nil in `mu4e-compose-mode-map`:
(define-key mu4e-compose-mode-map (kbd "C-c C-j") nil)
(Right now, I'm not sure if mu4e-compose-mode-map should be quoted
here or not. I use `bind-key` or usually the `:bind` keyword of
`use-package` to bind keys).
HTH
--
Joost Kremers
Life has its moments