Assume mozilla is your standard mail tool. :-) When composing a mail via right-clicking on a file and choosing "send to / email recipient", the normal compose window opens, but the "to:" field is not editable. (But you can choose the recipient from the address book) This happens sometimes (can't exactly define when, sorry). Closing the window and trying it again always leads to the correct function.
This is most likely just a flavor of one of the most reported bugs: http://bugzilla.mozilla.org/show_bug.cgi?id=82534 So, basically a dupe.
*** This bug has been marked as a duplicate of 82534 ***