Hi jitske,
The may sound strange but I think that sometimes when this happens, what is really going on is that it has sent the command to your mail program, and your mail program is trying to show a dialog message and ask you to click, but the dialog is invisible.
The next time this happens, try hitting enter or escape or selecting your mail client window, and see if your mail client is waiting for you to do something.
Or choose the alternate non-mapi way to send mail.