Occasionally, Google IPs will end up on Spam blacklists such as SORBS or CASA.
Unfortunately, in situations like these there’s not much Google can do. Google says filtering based on the RBL list of IPs alone is a bad practice, especially in the case of cloud messaging services like Google, however Google can’t tell recipients how to accept or reject incoming mail, or what third party services or databases to use to determine trust level or spam content of mail.
In the past Google has made efforts to request delisting from P databases, like SORBS and CASA, for listed IPs - however this takes a significant amount of resources and yields mixed results. Some public blacklisting services can take a long time and require several steps before delisting IP address; others will relist IPs within days or even hours of them being delisted; and yet others may simply not respond to Google's requests.
As a result Google has taken the policy to not follow up with public blacklisting services and as such they contact third party blacklisting we no longer request that specific IPs get delisted. You can visit https://support.google.com/a/answer/27642 for an article on blacklisted IPs on our helpdesk.
Spam sent through Google IPs is the result of a variety of factors. Most commonly spam is sent through compromised accounts or infected machines, and although we have very quick automated mechanisms to stop these in time through account suspension, outbound mail monitoring messages, and so forth, a small portion of all mail sent over the Google servers will always be missed. Despite this being an extremely small percentage some RBLs will blacklist an IP if even several spam messages are detected from the IP over a 24 hour period.
Google says that generally speaking, it's bad practice to base spam filtering on and RBL database alone as it means that the spam filtering isn’t taking anything else into account such as authentication or more importantly data points like message content or sending header details. The practice results in a large amount of false positives and negatives. Also, in relying on third party blacklists the mail server in question will essentially be rejecting any message from Google if it was being sent via a SORBS listed IP without checking the content.
More concretely, Google suggests that the recipient whitelist the address or IPs in question depending on the capabilities and flexibility of their mail solution. This will address the issue in the short term.
In the long term, Google suggests that the recipient mail servers look into more reputable sources of IP reputation verification so they can avoid relying on public blacklisting services entirely.
Message content based on Google ticket: 13298270
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