All Barracuda Spam Firewalls in use with a Microsoft Exchange server using Outlook Web Access, firmware versions 3.5 and above.
While incoming mail traffic typically uses port 25, users attempting to access their Outlook mailbox via Outlook Web Access (OWA) usually connect to a Microsoft Exchange server over ports 80 (TCP) and 443 (TCP). These connections should be made to the Exchange server, but if they must go through the Barracuda, the Barracuda can redirect them using port forwarding.
To configure this, navigate to the Advanced > Advanced IP Configuration (or Advanced > Port Forwarding) page and enter the source port (again, usually 80 or 443), the IP address of the Exchange server, and the corresponding port to use when connecting to the Exchange server. Then, you can redirect OWA traffic to the Barracuda and it will forward that to the Exchange server automatically.
The only catch is that the Barracuda uses port 443 as its own secure administration port, so if the users connect to OWA over port 443, you will need to change the Barracuda's secure port on the Advanced > Secure Administration page to something other than 443 to avoid a port conflict.
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