I'm sure it's not the answer you are looking for, but - do not use Exchange Online for sending bulk messages. Microsoft is very serious about limiting such scenarios, just days ago they also announced a tenant-wide limit: https://techcommunity.microsoft.com/blog/exchange/introducing-exchange-online-tenant-outbound-email-limits/4372797
So if you continue with this, you risk blocking your entire tenant, not just the user sending bulk messages. Use the alternatives mentioned in the blog post above, or any of the solutions out there that specialize in sending bulk/marketing messages.