We are updating our receiving limits in Exchange to help prevent attacks on your mail flow experience. Earlier this year in (February MC239262) we announced a stricter enforcement of our mailbox receiving limits. Taking your feedback into consideration, we are releasing an additional limit to block single-sender mail storms and deter DoS attacks.
Our mailbox receiving limits, as previously stated, apply to the messages received by a Microsoft Office 365 mailbox. If volume exceeds 3,600 messages in a given 60-minute window, the mailbox will no longer accept messages from the Internet, from other tenants, or from on-premises senders.