This solution applies to all Barracuda Spam Firewalls.
If all your outbound mail is being rejected on your Barracuda Spam Firewall with the error: 550 Unable to Relay, check to see if your mail server is listed in the Outbound SMTP Host / Smart Host field.
- In firmware version 3.x this can be found in Basic > Administration.
- In firmware version 4.0 this can be found in Advanced > Outbound.
- In firmware version 4.1 this can be found in Basic > Outbound.
Because your outbound traffic is configured to go through the Barracuda it will try to use the Smart Host of the mail server. Since the mail server is typically not configured to relay it rejects the messages with a "550 Unable to Relay" error.
Removing the mail server from the Outbound SMTP Host / Smarthost field will resolve the issue. Note: Do not remove Port 25. Remove only the IP address or host name of your mail server.
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