mm-handler parses its arguments from sendmail and builds a list of recipients and then processes that list in a loop. If it is really the case that a message addressed to, e.g., more than one list will be passed from sendmail to mm-handler as one invocation with multiple recipients, then the problem is you can only return one exit status, and the desposition might not be the same for all recipients.
The mailman.mc file (cited in README.mm-handler) mentions this vaguely. If the "m" flag is not present in the mailer definition (the "M" line) in sendmail.cf, then mm-handler will be run once per recipient.
It's important NOT to have an "m" flag, but just in case you forget that, mm-handler tries to handle multiple recipients a little.
-- -D. dgc@uchicago.edu NSIT University of Chicago