Email spamer Abuses Microsoft 365 Tenants by Relaying Through Proofpoint Servers

Proofpoint found spam campaigns were being relayed through a small number of their customers' email infrastructure through M365 relaying. The root cause of the issue was Proofpoints email routing configuration which, by default, allowed relaying of outbound messages from M365 tenants, but without specifying which M365 tenants to allow. There has since been a fix issued.