Quota's on mail

I just installed procmail on a server and it is deliverying mail just like
it should.  I would now like it to use the quota feature that it has.  I
have tried adding the M=<size> on the Mlocal line in the sendmail.cf file,
but it did not seem to make any difference.  Is there anything else that
has to be added to implement quotas?  Does the mail spool have to have a
quota monitoring program running or does procmail check the size of the
mail file for each user and take appropriate action?

Mike