SBS 2003 Cannot send mail-POP3 Connector

*Setup: Small Business Server with POP3 Connector configured for email. The POP email accounts are at RoadRunner. I am receiving email fine. I am using a dynamic IP address with an account and updater software that tracks the IP to the URL.
*Problem: When sending email I get the following errors – a System Admin NDR message that says the domain of the sender address @aroa.local does not exist. And the MSExchangeTransport has this error: A non-delivery report with a status code of 5.1.8 was generated for recipient rfc822; @austin.rr.com
*Question: It looks to me like RoadRunner is verifying the senders’ domain. Is there a way to make this work with the dynamic DNS setup? I am not familiar with MX records but it looks like at dyndns.org (where the account is) this might be an option.
Thanks…this is important and timely…