Hi @Darek L ,
Agree with michev that sending bulk email via Office 365 is actually not a supported use of the service.
According to the Exchange Online limits, "To discourage the delivery of unsolicited bulk messages, Exchange Online has recipient limits that prevent users and applications from sending large volumes of email. These limits are applied per-user to all outbound and internal messages." And as mentioned in the Note, it's recommended to use third-party providers that specialize in these services for your requirement to send bulk messages:
The following official document also mentioned that the recommended solutions are sending bulk messages through on-premises email servers or a third-party bulk email provider:
Recommendations for customers who want to send mass mailings through EOP
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