It is no secret that Google has the best anti-spam email filtering on the market. After purchasing Postini, Google lowered their prices for inbound filtering to only $3 per user per year. What an incredible deal!
So say you have a mail server with poor anti-spam filtering and you want to move to Exchange 2007 which has pratically zero out-of-the-box anti-spam features, then you need to go with some third party. I propose that Google’s Postini not only is suitable choice to act as a filtering gateway, but it can also be a critical piece to factor in for a migration from one email platform to another.
When going through the activation for Google’s Postini, you will have to add 4 MX records for Postini which point back to your Exchange server. So this is what I suggest for a configuration during a transition phase:
MX #1 – current mail server (priority 10)
MX #2 – postini 1 (priority 20)
MX #3 – postini 2 (priority 30)
MX #4 – postini 3 (priority 40)
MX #5 – postini 4 (priority 50)
MX #6 – exchange server/other mail server (priority 60)
Then, add paid users to Postini. When all the users are configured on Postini, switch MX prioritization so that Postini occupies the first 4 slots and leave your old mail server to the final slot (priority 60).
MX #1 – postini 1 (priority 10)
MX #2 – postini 2 (priority 20)
MX #3 – postini 3 (priority 30)
MX #4 – postini 4 (priority 40)
MX #5 – new exchange server/other mail server (priority 50)
MX #6 – old mail server (priority 60)
Paid users on Postini will be filtered and arrive in Exchange while unpaid users will also arrive in Exchange but will not be filtered. Keep the old mail server up for a week or two until you confirm that everything is working smoothly. Hope this helps.