Understanding Rate Limits for Microsoft Send Mail API

Sending Limits Overview

Sending limits for Microsoft Send Mail API encompass the number of recipients, messages, and recipients per message that a user can send from their Exchange Online account. These limits play a crucial role in maintaining a balanced and efficient email sending environment.

Recipient Rate Limit

The recipient rate limit is an essential aspect of Exchange Online's email policies. It serves as a deterrent against the distribution of unsolicited bulk messages. These limits are imposed on a per-user basis for all outbound and internal messages. It's important to note that legitimate bulk commercial emails, like customer newsletters, should be directed through specialized third-party providers.

Recipient Limit

The recipient limit is a key factor in determining the maximum number of recipients allowed in the To:, Cc:, and Bcc: fields of a single email message. Distribution groups in the organization's shared address book count as one recipient, while personal distribution lists count each recipient separately.

Recipient Proxy Address Limit

The recipient proxy address limit dictates the maximum number of aliases (email addresses) that a recipient mailbox can possess. This limit ensures that the system resources are optimally utilized and that excessive aliases are avoided.

Message Rate Limit

Message rate limits define the quantity of messages a user can send from their Exchange Online account within a specific time frame. This mechanism prevents overconsumption of system resources by a single sender. Messages submitted at a rate surpassing this limit via SMTP client submission will be rejected, necessitating reattempt from the client.

Sending Limits Based on Plans

The sending limits vary based on the Microsoft 365 plan. For Microsoft 365 Business Basic and Standard, Microsoft 365 Business Premium, Microsoft 365 Enterprise E3/E5, Office 365 Enterprise E1, Office 365 Enterprise E3/E5, and Office 365 Enterprise F3, the recipient rate limit is 10,000 recipients per day. The recipient limit is customizable up to 1000 recipients. The recipient proxy address limit is set at 300 aliases. The message rate limit is 30 messages per minute.

Recipient Limit for Encrypted Messages

For encrypted messages, the recipient limit is approximately 200 recipients. This limitation considers the length of the email address and the size of the encryption policy generated. Maintaining this limit ensures efficient handling of encrypted communication.

Conclusion

Understanding the rate limits for Microsoft Send Mail API is essential for effective email management and delivery. By adhering to these limits, users can ensure optimal system performance, prevent spam, and create a smooth communication experience for all parties involved.