Few weeks back I wrote a short article about the improvements Microsoft has recently made in the process of delisting blocked IP addresses in Office 365/EOP. The newly introduced self-service portal (https://sender.office.com/) was targeted to user outside of Office 365 however. As I briefly mentioned in that article, Microsoft was also preparing a self-service portal targeted at Office 365 admins.
This portal is now available as part of the Exchange Admin Center in Office 365 and it’s branded as the EOP Action center. To access it, go to the Office 365 portal -> Exchange -> Protection -> Action center. The delist process itself is very simple: find/select the address and press the Unblock link on the right. As with the “external” self-service portal, some limits apply and you will not be able to delist the same address/user more than a handful of times. After that number has been reached, you will have to contact support.
Here’s again the list to the TechNet article about the new EOP Action center, with detailed steps on the process: https://technet.microsoft.com/en-us/library/dn458545(v=exchg.150).aspx