[chrismartin]

The pattern -- and volume -- of messages that Gmail would receive from an independent server with legitimate users would look very different than someone running a similar server to send spam.

Perhaps a spammer with a new server could gain Gmail's trust by passing genuine-looking correspondence, but this would stop working as soon as they start to send bulk messages.

Seems to me that tuning Gmail's filters to recognize and trust small-volume email servers would not be difficult or time-consuming.

[fauigerzigerk]

That could well be. It's hard to know without access to the data, but they could probably do better if they tried.

However, I fear if Google were to try harder, spammers would also jump on that opportunity. It's easier to mimic the patterns of a small legit SMTP server than to pose as one of a few big well established email providers with known IP ranges. They could potentially use millions of compromised PCs, each sending spam in low volumes mixed with non-spam traffic.