Mail delviery blocked

Your tenant still cannot send after a spam incident, despite an engineer setting outbound limits. Those limits alone do not identify or remove the underlying restriction. First capture the complete NDR from a fresh test, including its diagnostic code and UTC time. A 5.1.8 restriction usually concerns an individual sender; after securing that account, an authorized administrator can review it under Restricted entities in Microsoft Defender. Tenant-level 5.7.705 or 5.7.708 blocks require Microsoft Support after the compromise or open relay is resolved. Review affected credentials, revoke sessions, enforce MFA, and inspect forwarding rules and connectors before requesting removal. Update the existing support case with the NDR, affected users, remediation completed, and two weeks of business impact. Request a callback through the Microsoft 365 admin center, referencing that case. The forum cannot inspect or lift your tenant’s block, and I cannot promise a restoration time.