I don't think any of the commercial services will send messages without an unsubscribe option on each message, as the risk of having their servers flagged as sending spam is too great.
Even if your list subscribers are already registered as members of your forum, is there any harm in giving them the option to unsubscribe if they don't want to receive your emails any longer? Trying to force them to stay on the list just means they're going to start flagging your messages as spam rather than unsubscribing - either way, they'll eventually stop reading.
If you're willing to have the unsubscribe instructions, check out AWeber - they've got plans that start at $19.95 per month for up to 10,000 subscribers.
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