Note that there is apparently an option in phpBB to send via SMTP as well (which would be Ok, though if you're doing this often or to a really large group of people, we may still ask you to take steps to limit your resource consumption).
This is ok. I suggest using smarty.dreamhost.com as the SMTP server (not mail.[your domain] (though this might change at some point).
As long as you're making it extremely clear to people when they sign up that they will be receiving messages, as long as you are using a confirmed opt-in process (which apparently phpBB does), you should be ok... just make sure to read our policy and clarify any potential issues /before/ they come up.
Again, make sure you configure it to set the envelope-sender address if it doesn't do this by default.
Lastly, we haven't announced this yet, but we are planning on improving the existing announcement list functionality... if you have suggestions for improvements, post them to a new thread or to the suggestions area, and we'll make sure they get to the attention of the right people.