i just looked up your domain. You should setup DKIM and SPF records for your domain. Don't have the links to explanations handy, but basically those are two different technologies that allow you to authenticate the source of the mail thereby proving that a spammer didn't just set the from field to your domain. It's quite easy under Google apps. DKIM requires you to generate a key for your domain which you enter as a text record at your DNS provider. SPF doesn't require a key, just a DNS txt record telling the receiver that he should check with Google that this is valid user.
Hope it helps, Tsvi
