I have a Postfix Mail Server running on Unix FreeBSD 8.1-RELEASE on i386. The mail system is POP3 mail. It is running behind a Barracuda Firewall. I have 4 branches and the HQ. The HQ and the other bigger branches are connected through a VPN Link from an ISP. The other 3 branches are connected through Fixed Wireless Broadband and they are supplied by another ISP. All emails are configured on Outlook. The HQ and the other branch connected through VPN Link have no problems with sending/receiving emails internally/externally. The three branches that are using Fixed Wireless Broadband are able to send/receive emails internally and receive external emails. But they cannot send external emails. It stays at the outbox and returned with error "Relay Access Denied". Can someone help me on this issue? I am quite new to to Unix and Postfix Mail server and in deep problem.