Details specific to the Mailman available for those using Dreamhost ISP. As of this writing (20220315), it is version 2.1.23.

You can set Mailman to allow use of Topics.

Setting up topics:

1. Go to Configuration Category “Topics”
   1. Enable
   2. Specify number of body lines to scan for the topic, 5 is default. If 0, then only Subject is scanned.
   3. Topic Name. Meaningful name for the admin
   4. Regexp. A word, or regex to match. E.g., one (matches messages with the word “one”), [two] (matches messages with “[two]”, “three|four” (matches messages with either a “three” or a “four”)
   5. Description. Write a book if you want to describe this topic.
   6. Add new item… and …after this one: click to add new topic, could be after (or before) this one.

Using Topics.

• Default. Everyone gets all messages regardless of whether it's on any topic or not.

Each user can change this in their membership settings. The Admin can also set these for any user (membership management, click on user email). Scroll down and set:

• Which topic categories would you like to subscribe to? Select those you are subscribing to.
• Do you want to receive messages that do not match any topic?
  • No. You will only get messages on those topics you've subscribed to. (If you haven't subscribed to any topics, you'll get all of the messages sent to the list regardless of any topic.)
  • Yes. You will get all messages sent to the list whether or or not on any topic you may have selected.

Examples. Assume a topic is configured as “ONE”. This means for examples, that a message with the subject “This is one message” will be considered in that topic and a message with subject “This is message two” will not be in the ONE topic.