Google Apps allows for the use of inbound mail gateway servers which admins can leverage to process their inbound mail in different ways―such as archiving it or filtering out spam―before passing it onto the Google Apps mail server for delivery.
Today’s launch adds more flexibility for Apps admins when using inbound mail gateway servers:
- Multiple upstream server systems – to accommodate more complex network architectures, customers can now have multiple server systems upstream from Gmail.
Message tagging – an upstream system can now tag messages as spam in the header, which Gmail will honor, allowing for full delegation of spam filtering to a third party solution if desired.
Reference By Google