Honestly, this sounds like it'd solve the majority of the Gmail Fail stuff I've been seeing, and with Google's ability to collect store the "triplets", they should be able to make this all pretty transparently from its users' perspectives.

Greylisting: Whitepaper:

The Greylisting method is very simple. It only looks at three pieces of information (which we will refer to as a "triplet" from now on) about any particular mail delivery attempt:

1. The IP address of the host attempting the delivery
2. The envelope sender address
3. The envelope recipient address

From this, we now have a unique triplet for identifying a mail "relationship". With this data, we simply follow a basic rule, which is:

If we have never seen this triplet before, then refuse this delivery and any others that may come within a certain period of time with a temporary failure.

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