I remember reading about attackers a while ago that broke into Gmail accounts of webmasters.
Instead of doing all kinds of things there they simply added mail forwarding filters to the account.
Why did they do that? To suppress email notifications of domain transfers. Registrars sent an email to the registered owner if a domain transfer is initiated. They may also sent emails in other situations, for instance when the domain is about to expire or if important information change.
Google seems to believe that it is time for all users to review their mail forwarding and delegation settings on Gmail. From today on a message will be displayed asking the user to review the filters set for the Gmail email address.
Users who do not have filters setup won't get the message, obviously.
The message reads: Your filters are forwarding some of your email to [email address]. Options are to review the settings and a link to information about the notification.
Users who do not get the notification message just yet can verify their filtering settings manually instead. A click on the preferences icon in the top right corner, and the selection of Settings from the context menu that opens up loads the options.
A click on Filters opens the filtered mail listing. You can review all filters that are currently configured right there. These filters can be modified or deleted if need be.
You furthermore should click on Forwarding and POP/IMAP to see if the configuration there is correct. Check if mail is forwarded and whether POP or IMAP access are enabled. You may want to disable forwarding or external mail access if you do not need it.
Lastly, you need to click on Accounts and Import to make sure that no one has been granted access to your account who should not have it.
I personally have not seen the filtering notification yet, even though filters are configured in the account. The announcement on the official Google Mail blog suggests that the message will appear for about a week before it will stop appearing. (via)
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