This is probably a fairly isolated bug, which is dependent on having a very particular setup of server and mailman, but I'll write it up as it took me a long time to track down.

Using this module, emails sent to the request list were always disguarded by mailman.

It seems that the problem was that my host (1and1) marks all email sent from scripts with 'Precedence:bulk' in its headers. This makes some mailman servers reject the requests from drupal as it considers them to be spam.

The simple solution for this is to set the 'Precedence' header to nothing in _mailman_manager_subscribe() as shown below:

$mailsuccess = drupal_mail('mailman-manager', $commandaddress, '', $command, variable_get('site_mail', ini_get('sendmail_from')), array('Precedence'=>''));