SMTP for phpBB

I recently moved my home-based website/forums to DreamHost. Since then, I don’t think any of my members have been receiving their emails. I want the email to come from my (dreamhost) email address, which works fine. I can’t seem to add a user or use the mass email function, I get the following error :

Ran into problems sending Mail. Response: 554 : Relay access denied

I’m quite new at this, how do I set this up so that mail gets through? Thanks.