Dear colleagues, 

Microsoft has announced the rollout of the External Recipient Rate Limit (ERR) for Exchange Online. This feature is designed to limit the number of external recipients a mailbox can send messages to within a 24-hour period. This change is designed to support responsible usage and ensure fair resource usage within Exchange Online. 

What is changing 

Currently, each mailbox in Microsoft 365 can send emails to up to 10,000 total recipients in a 24-hour period. Under the new policy, Microsoft will apply an additional sub-limit specifically for external recipients (addresses outside the university’s email domains): 

• • New Limit: Each mailbox can send to up to 2,000 external recipients in a 24‑hour window.  

• • Internal emails: (to addresses ending in @utoronto.ca, @toronto.edu, or other approved university approved domains) are not affected. 

• • Once a mailbox reaches the 2,000 external recipients, any further messages to external addresses will be temporarily blocked until the 24-hour window resets. Internal messages can still be sent normally. 

This change is enforced by Microsoft and cannot be disabled. 

Action required 

• • Review your current mailing practices. 

• • If your mailbox needs to send more than 2,000 external recipients in 24 hours, use approved mass-mailing tools (i.e. SendGrid, Constant Contact, or Azure Communication Services for Email) designed for high-volume external communications.  

• • Learn more: Exchange Online to introduce External Recipient Rate Limit 

Timeline 

• • October 1, 2026: ERR enforcement begins for all existing tenants. 

If you have any further questions or concerns, please submit a ticket to the Enterprise Service Centre at https://uoft.me/m365help. 

Kind regards, 

Enterprise Applications & Solutions Integration (EASI)
Information Technology Services, University of Toronto
215 Huron Street, Toronto