Of the four classes you listed...
* User -- A user, what lists they're signed up to, what messages they've received, methods to subscribe, unsubscribe, deactivate

* List -- Description, what messages are attached, what users are subscribed, create a new list

* Message -- Includes subject, body, schedule, has method to populate message queue with record for all users who should receive it

* Queue -- Send or drop pending messages, list sent or pending messages

I'd create (at least) a 5th. Probably a 6th as well.

5th would be Messages (which might be your queue) It's a collection of messages.

6th would be MessageBO (MessageBO). It would know how to take a message off a Messages(Queue) and send them. It would be responsible for the bulk of actually pushing a message off to SMTP.