We route as much mail as possible through Postini, even for domains that don't actually use Postini's filtering service. Ultimately, we intend to route all mail this way. Postini provides us with protection against directory harvest attacks even when the filtering is turned off, thus reducing the load on our server and improving performance for all customers.

When you ask us to disable Postini, what we really do is disable filters on your existing mailboxes, and reconfigure your domain to use a disabled "template" account. When you create a new account on the mail server, Postini will create a corresponding account the first time mail is received for the new address. It will, as usual, send a welcome message to this new address, even though the account has been created in a disabled state. There isn't any way to disable this message.

So essentially, you receive this message because there's no way to prevent it from being sent — but it can be ignored, and you won't be charged for the account.