Sending Limits from Exchange on prem to Exchange Online

Question

Hello, we need to send 18,000 messages to 18,000 individuals from a system on premise to users in the cloud. This needs to be done all at the same time. Will the limit of only being able to send 10,000 messages a day affect an on premise system? Would our on premise system just work like any external sender, our mail is currently routed to exchange on prem and then onto exchange online via the connector.

Answer 1

Yes,: Additionally, one sender can only send 3600 messages per hour:
https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#receiving-limits

You should create a Distribution Group instead , add those users and sent to that group - which will only count as one recipient

https://learn.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits#sending-limits