Sieve support
For each mailing list generate a Sieve script that would erejecg
a message, that mailman will bounce or discard.
Upload the script over ManageSieve any time the script is supposed to change (subscriber added, list configuration changed).
This way MTAs can evaluate the Sieve scripts before closing the SMTP connection and rejecting the message at that level. This leads to less backscatters.
Introduce a new configuration option stating which capabilities the ManageSieve server supports.
The scripts are very powerful: if the mail has high spam probability according to the message headers, the content of the headers with the matched test can be included in the smtp rejection. Or a message to listname-LEAVE can be rejected, if the sender is not subscribed to the mailing list and therefore cannot be removed from there.