If you ever see this message in your /var/log/qmail/current (or wherever you put it):
alert: cannot start: unable to open mutexit can mean a couple of things, but as the message suggests, it surely indicates some file-access problem.
Google was not of any help there, but strace helps to find out that it tries to access file lock/sendmutex, so all you need to do is to recreate it, and set the owner to qmails:qmail.
On the matter of why this happened at all, I have a fairly simple explanation: I have been HUP-ing and KILL-ing qmail-send quite agressively, while doing some testing.
Unfortunately, I may have missed some mail during these ten minutes, so if I don't respond to you in the near future, perhaps that's what happened: please resend it.