When using the Symantec Messaging Gateaway alias feature together with the subaddress removal feature, Recipient Validation fails for the aliased email address. This can result in the message being rejected or silently deleted depending on the configured action for Recipient Validation.
550 5.1.1 Recipient address rejected: User unknown
This issue has been addressed with the Messaging Gateway 10.6.2 release.
Please see the Messaging Gateway Administration Guide for detailed instructions on how to update the SMG software.