Have you had problems receiving e-mail from Gmail users recently? If so, you're one of thousands. Over the past month, major anti-spam vendors have had to apply scrutiny to Gmail in a way they haven't had to before, and the result is reduced delivery performance and sometimes outright blocking of Gmail. Some messaging hosts are being instructed to reject SMTP connections from Google. Ars Technica has independently confirmed this.

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.

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http://arstechnica.com/security/news/2008/04/gmail-being-throttled-blocked-by-some-anti-spam-vendors.ars