If you have not already done so, I highly recommend you create an SPF record for your domain as this will make it much more difficult for spammers to forge your domain in order to spam domains in other organizations.

Sender Policy Framework... WikiPedia Definition

Creating your own SPF record is a relatively simple process, Microsoft even provides a web-based GUI wizard that will help you do this.. Domain holders need to complete an inventory and publish all IP addresses of their outbound e-mail servers in the DNS zone file. This is an administrative step that requires no changes to an organization's e-mail or DNS software. Even if your domain has no outbound e-mail servers, you can help protect yourdomainfrom spoofing by publishing anSPF record in the DNS that states this. Microsoft Safety Home Page: click here.

Implementation Tips for the Sender ID Framework—Creating Your SPF Record... Click here.

Sender ID Overview: Click here.