Login to Microsoft 365 Defender: Policies & rules --> Threat policies --> Anti-spam policies
https://security.microsoft.com/antispam

Edit the "Connection filter policy (Default)" or make a new rule to allow the IP address(s).

An example error:
Message Delivery Failure. Attempt (0): Could not connect to mail server for domain. The remote mail server could not be contacted at this time. Message has been requeued.

Keywords: Email, E-Mail, 365, Hosted, Whitelist, Allow