Since the CAPTCHA for Google's Gmail e-mail service was defeated in February some anti-spam vendors have taking measures. In some cases this goes even as far as to reject SMTP connections from Google.
It all began when Google's bot-busting CAPTCHA for Gmail was defeated sometime in February. According to sources around the anti-spam industry, the result has been a marked increase in spam originating from Gmail SMTP servers. Some say the spam increase started even earlier, but all are in agreement on one thing: this is a serious problem.
A support analyst with MessageLabs, a major provider of software-as-a-service anti-spam filtering, told Ars Technica that "some spammers have hacked into the Gmail captcha system, and were able to relay spams appearing to come from Googlemail's IP addresses. This has caused many IPs of theirs to appear to be sources of spam." For their customers, this means a decrease in performance. "We have a traffic-shaping system that throttles IPs that we believe to appear to be a source of spam. The result is that for the past couple of days we have been seeing issues like this with Gmail," the analyst concluded.
Later, once we confirmed that their servers were not only slowing the delivery of mail but sometimes outright refusing connections to some Gmail relays, another support analyst told us that "Gmail has sent out a lot of spam recently, and they are registering on our traffic shaping system." When spam is sent out over a Gmail relay, that relay can sometimes get completely blocked, causing problems for thousands of legitimate Gmail users. MessageLabs then has to play whack-a-mole, trying to throttle only the SMTP relays that are spamming.