Here's the official response from Brett at DreamHost...
Hope it's of use to others.
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We provide you with Mailman, an open-source mailing
list program. You can find more information about
Mailman (documentation, etc.) at http://www.list.org/
Currently we offer 10 lists for an additional monthly cost of $4.95 per month on each of our plans. We do not limit the number of subscribers each list may have. Please not also that we do not limit the size of messages, though all messages will pass through your DreamHost disk space, so you need to ensure that enough space is available to handle
large attachments. Mailman has its own web-based control panel that enables you to control list membership, subscription requests, and just about anything else that you can think of.
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