Exchange Online to introduce External Recipient Rate Limit

Update 12/16/2024: After carefully considering your feedback and understanding the need for additional time to adjust, Microsoft has decided to delay the rollout of the cloud hosted Mailbox External Recipient Rate Limit (MERRL) by about 6 months. Please see below for new dates.

Today, we are announcing that, beginning in October 2025, Exchange Online will begin enforcing an external recipient rate limit of 2,000 recipients in 24 hours.

Exchange Online does not support bulk or high-volume transactional email. We have not enforced limiting of bulk email until now, but we plan on doing so with the introduction of an External Recipient Rate (ERR) limit. The ERR limit is per user/mailbox and being introduced to help reduce unfair usage and abuse of Exchange Online resources.

This new limit is not going to be applicable to the following Microsoft integrations until further notice:

• Server-side synchronization between customer engagement apps and email servers - Power Platform | Microsoft Learn
• Outlook.com - Connectors | Microsoft Learn

What about the Recipient Rate Limit?

Exchange Online enforces a Recipient Rate limit of 10,000 recipients. The 2,000 ERR limit will become a sub-limit within this 10,000 Recipient Rate limit. There is no change to the Recipient Rate limit, and both of these will be rolling limits for 24-hour windows. You can send to up to 2,000 external recipients in a 24-hour period, and if you max out the external recipient rate limit then you will still be able to send to up to 8,000 internal recipients in that same period. If you don't send to any external recipients in a 24-hour period, you can send to up to 10,000 internal recipients.

For example:

• You use a cloud-hosted mailbox to send to 1,000 external recipients and 2,000 internal recipients at 6:00AM on Day 1, for a total of 3,000 recipients. You then send to another 1,000 external recipients at 8:00AM on Day 1. Because you sent to 2,000 external recipients, you will be blocked from sending to external recipients until 6:00AM on Day 2. During this period, you are able to send to up to 6,000 internal recipients, but for this example, let's assume that you don't.
• At 6:00AM on Day 2, the 1,000 external recipients and 2,000 internal recipients sent at 6:00AM on Day 1 will no longer count toward the 24-hour limit calculation. Thus, from 6:00-8:00AM on Day 2, you can send to up to 9,000 total recipients (e.g., 9,000 internal or 8,000 internal plus 1,000 external).
• If you don't send to any internal or external recipients during this period, then at 8:00AM on Day 2 you can again send to 10,000 recipients total with up to 2,000 of them being external recipients.

How will this change happen?

The new ERR limit will be introduced in 2 phases:

• Phase 1 - October 2025, the limit will apply to cloud-hosted mailboxes of trial tenants and all cloud-hosted mailboxes of tenants created after that date. This phase will include an Exchange Admin Center mail flow report detailing the External Recipient count of cloud-hosted mailboxes belonging to the tenant.
• Phase 2 - April 2026, we will start applying the limit to cloud-hosted mailboxes of existing tenants.

Please refer to the ACS email documentation (Prepare an email communication resource for Azure Communication Services - An Azure Communication Services concept article | Microsoft Learn) or reach out to your Microsoft account team for further support in this journey.

Q&A

What are the options for customers who have business needs that exceed the ERR limit?

If you have a cloud-hosted mailbox that needs to exceed the ERR limit, you can move to Azure Communication Services for Email, which is designed specifically for high volume email sent to recipients external to your tenant.

Is this limit counted against per recipient or per unique recipient?
We will be counting per recipient not per unique recipient. So, if you sent 100 emails to the same 5 external recipients, it would count as 500 external recipients.

Exchange Online Transport Team