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You can read more about CSV in this forum thread.
In summary - this change will help stop some spam appearing to come from domains FastMail owns or domains hosted with FastMail where you have chosen to use our DNS servers (ns1.messagingengine.com and ns2.messagingengine.com) to handle your entire domain.
If you are running your own DNS, you can read the thread linked above for ideas for adding your own CSV records.
We don't anticipate any problems related to this change, as we have whitelisted all the hosts we use for sending mail, but if you do see anything change, please report it to support or to the forum thread.