mark for something uses insert key. This isn't a great default since many laptops do not have this key.
mark for something
What would be a better key?
dired/sunrise-commander use 'm' for marking, but that's already taken in mu4e; sunrise-commander has INS as an alternative to 'm', which why mu4e got that, too.
Perhaps 'M'? or 'X', which is related to 'x' which does the action.
I'd personally love to see an overhaul of the default mu4e bindings so that they're consistent with other standard emacs packages, most particularly dired and ibuffer, ie. "m" for Mark, "D" to delete, "x" to execute marks, "% ..." to mark things matching patterns. It would make life much easier for many new users, who are likely to have developed muscle memory for dired.
And while we're at it, org-based keybinding too, so C-c o could open a URL, instead of M-enter, etc. But really, just keys that exist (on all systems) would be nice :).
It appears the * (asterisk) key is bound to this function on OS X. Works for me.