> > What would generate the 'Too many recipients.' error?
> 
> MailMan ouputs messages with (guesstimate) 100 recipiants per
> message.  If the MTA that MailMan is talking to has such a limit,
> you'll get that error.

Isn't this defined in the privacy options under the "Ceiling on acceptable
number of recipients for a posting" section?

   max_num_recipients (privacy): Ceiling on acceptable number of
   recipients for a posting.

   If a posting has this number, or more, of recipients, it is held
   for admin approval. Use 0 for no ceiling.

I typically have mine set to 10, which is the default shipped with
Mailman.  You can also override the default with

   DEFAULT_MAX_NUM_RECIPIENTS = n

in $prefix/Mailman/mm_cfg.py

Chris