certificate for exchange

Hello, I run SBS2003 w exchange for a very small company. right now, i cant set up outlook to get email out of the network because the certificate is not from a trusted issuer. Outlook works in the network just fine, just not out.

With that i just dont know anything about certificates. I dont want to buy anything i dont need, at the same time, make sure I get whats expected for outllok to accept the proxy connector.

Please help. thank you!