Sending limits apply to the number of recipients, number of messages, and number of recipients per message that a user can send from their Microsoft 365 email account.
Recipient rate limit: 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.
Recipient limit: This is the maximum number of recipients allowed in the To:, Cc:, and Bcc: fields for a single email message.
Note: For AD Email-enabled distribution groups, the group is counted as one recipient. For distribution groups stored in the Contacts folder of a mailbox, the members of the group are counted individually.
Message rate limit: Message rate limits determine how many messages a user can send from their Exchange Online account within a specified period of time. This limit helps prevent the overconsumption of system resources by a single sender. If a user submits messages at a rate that exceeds the limit via SMTP client submission, the messages will be rejected and the client will need to retry.