Microsoft Office 365 email limits

Microsoft’s email service has a number of documented limits that, when exceeded, will result in account suspension for 24 hours. This feature is intended to limit the impact of email abuse.

Account Limits

Microsoft’s email service has a number of documented limits that, when exceeded, will result in account suspension for 24 hours. This feature is intended to limit the impact of email abuse.

Details at https://docs.microsoft.com/en-us/office365/servicedescriptions/exchange-online-service-description/exchange-online-limits

Sending limits

Sending limits apply to the number of recipients, number of messages, and number of recipients per message that a user can send from their Exchange Online (Office 365 Email) account.

For distribution groups stored in an organization's address book (Exchange Distribution Lists), 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.

Recipient rate limit: 10,000 recipients per day per user

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.

Note: After the recipient rate limit is reached, messages can't be sent from the mailbox until the number of recipients that were sent messages in the past 24 hours drops below the limit. For example, a user sends an email message to 5000 recipients at 09:00 a.m., then sends another message to 2500 recipients at 10:00 a.m., and then sends another message to 2500 recipients at 11:00 a.m., hitting the limit of 10,000 messages. The user won't be able to send messages again until 09:00 a.m. the next day.

Recipient limit: 500 recipients per single message (can be increased to 1,000 max)

This is the maximum number of recipients allowed in the To:, Cc:, and Bcc: fields for a single email message.

Message rate limit: 30 messages per minute per user

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 over consumption of system resources by a single sender.

Note: The limit only applies for email clients using legacy SMTP protocol to submit email. It does not affect Outlook as configured by IT staff on the campus.

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.

Reference link:

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