Message submission rate - Avoid spam sending

1. Check the message submission rate limit: Login to Exchange Administrator Center
Navigate to Mail Flow > Connectors and check the rate limit for sending connectors.
By default, Exchange Online has a send rate limit of 30 messages per minute.
2. Use Bulk Mail Service: If you need to send a large number of emails, it is recommended that you use the Bulk Mail Service (Microsoft 365's Bulk Mail feature).
In the Exchange Administrator Center, navigate to Mail Flow > Bulk Mail to configure bulk mail delivery.
3. Configure Send Connectors: In the Exchange Administrator Center, navigate to “Mail Flow” > “Connectors”.
Create or edit the Send Connector to adjust the Rate Limit and Retry Interval.
4. To adjust rate limiting using PowerShell: Open the Exchange Management Shell (with administrator privileges).
Use the following command to adjust the rate limit for the Send connector:
powershell
Set-SendConnector -Identity “YourConnectorName” -MaxMessagesPerConnection 100 -MaxMessageRatePerMinute 100
5. Avoid being flagged as a spammer: Ensure that the content of the email complies with the anti-spam policy, avoid using spam keywords, configure SPF, DKIM and DMARC records, and ensure that the email is authenticated.
Use mailing list management tools (Microsoft 365's mailing list feature) to avoid sending to invalid or unsubscribed mailboxes.
6. Monitor email delivery status: In Exchange Administrator Center, navigate to “Mail Flow” > “Email Tracking”.
Check the delivery status of your emails and check for emails that have been marked as spam.