The Digest options allow you to determine if, and how, Mailman will control batched-delivery options for your list.
The digestable option controls whether users can choose to receive your mail in digest (collected batches of messages) as opposed to immediate delivery of each message.
The mime_is_default_digest controls whether a plain text format or one that allows other MIME types is the default format sent. MIME (Multipurpose Internet Mail Extensions) is an Internet standard that extends the format of email to support non-ascii characters, multi-part attachments, and non-text attachments.
The digest_size_threshold tells Mailman how large it should allow a digest to get before sending it out.
The digest_send_periodic tells Mailman whether or not it should send the collected messages even if it hasn't reached the size listed in digest_size_threshold.
The digest_header and digest_footer options allow you to add a header or footer (or both) to each digest sent out. By default, digests have the Mailman list's name (real_name), the address of the list, and a link to the listinfo page for the list are added to the bottom of every digest.
The digest_volume_frequency option controls how often a new volume should be started. When a new digest volume is started, the volume number is increased by one and the issue number is reset to one. A list with a monthly digest_volume_frequency would change to Volume 2, Issue 1 with the first digest sent in it's second month.
The new_volume option instructs Mailman to immediately start a new volume. This may be handy if you wish to end a volume right before a long break so any messages sent during the break would go into a new volume. This option doesn't change a permanent setting, but performs an immediate action instead. This option will not effect the delivery of a digest.
The send_digest_now option instructs Mailman to immediately send a digest of all collected messages. This option is allows you to force out a digest (instead of waiting for the daily or size-limit based sending). This is very handy if important information needs to be sent out in a more timely manner. This option doesn't change a permanent setting, but performs an immediate action instead. This option will not effect the volume of a digest (although it will effect the issue number).