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Original KB number: 3013740
Problem
An external email server can't connect to Microsoft Exchange Online Protection because its IP address was added to a blocklist. When a sender tries to send mail through that server, the sender receives a non-delivery report (NDR) that contains the following error message. (The message doesn't include the name of the specific blocklist.)
5.7.1. Unable To Relay: Blocked by Customer Allow list
Cause
This issue occurs if the IP address is added to both the incoming connector and the IP Allow list on the Connection Filtering page of the Exchange admin center. In this scenario, the NDR doesn't include the name of the specific blocklist if the IP address is added to a real-time blocklist (RBL) that's used in Exchange Online Protection.
Solution
To determine the name of the specific blocklist that's preventing the IP address from connecting to Exchange Online Protection, remove the IP address of the external email server from the IP Allow list. After you do it, the NDR will include the name of the specific blocklist.
More information
Ideally, only on-premises IP addresses should be added to connector settings.
For more information about 5.7.1 NDR errors, see Fix email delivery issues for error code 550 5.7.1 in Exchange Online.