Hi @Algoarmor ,
According to my research, this error may occur when your tenant is new or has a trial license, and the IP address you are using is blocked by Microsoft for security reasons. To fix this issue, you may need to contact Microsoft support and request them to add your tenant to a sending exclusion for 60 days12. You can raise a support ticket from https://aad.portal.azure.com.
Alternatively, you can try to use a different IP address or network to send emails from your group.
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