It has come to our attention that someone using our Secondary MX service to back up a Barracuda filtering appliance is not whitelisting our servers; rather they are allowing the Barracuda to report our servers as a spam source. We do not support using our Secondary MX service with filtering disabled against another filtering appliance or program as this will lead to undesirable reporting results.
In this case, the result of allowing mail to pass unfiltered to a Barracuda has landed our mail servers on their blacklist. Unfortunately there is nothing we can do to prevent this. We have always recommended that our servers must be added to the primary mail server's whitelist. Fortunately, this does not effect our ability to deliver mail since we maintain a strict separation between incoming and outgoing mail paths.
It is quite common for end users to report our Secondary MX and SMTP Redirection services as a spam source. This is always incorrect because our mail servers only deliver and process mail as they are configured by the account control center. For this reason, we do not recommend additional filtering mechanisms for SMTP Redirection domains unless it is configured to always accept mail from our service. If you run filters on your mail server (common for Secondary MX), it is your responsibility to configure your mail server and/or filtering system to properly handle these messages.
Please remember to always whitelist our servers to ensure proper delivery and processing of your mail. We most certainly do support using our Secondary MX service with competing offerings - it is always wise to diversify resources that are critical to you - and remember to add our servers to their whitelisting mechanisms. If you have any questions, contact our technical support.