Microsoft have announced a significant change to how outbound email is managed in Exchange Online, introducing Tenant External Recipient Rate Limits (TERRL).
How have the limits changed?
Historically, Exchange Online relied on per-mailbox daily send limits (known as the Recipient Rate Limit or RRL). However, this approach didn’t fully address bulk or high-volume email scenarios at the tenant level. With the introduction of TERRL, Microsoft now enforces tenant-wide limits based on the number of email licences held. These limits apply to external recipients – defined as email addresses outside of the tenant’s accepted domains – and are calculated using a 24-hour sliding window.
These changes are in addition to the External Recipient Rate Limit covered in our previous blog post back in January.
What are the new email limits?
For Trial tenants, the limit is 5,000 total external recipients per day, regardless of licence count.
For Non-trial tenants, the daily outbound limit is based on the tenant’s email licences (any Exchange Online or Exchange Online Protection licence). More licences increase the number of external recipients your tenant can send to per day, but at a decreasing rate per licence. Your limit can be calculated by using the following formula:
500 * (Number of Non-trial Email Licences^0.7) + 9500
To make this easier to understand, here are some sample licence counts and the equivalent limit:
| Number of Non-trial Email Licences | Tenant External Recipient Rate Limit |
| 1 | 10,000 |
| 2 | 10,312 |
| 10 | 12,006 |
| 25 | 14,259 |
| 100 | 22,059 |
| 1,000 | 72,446 |
| 10,000 | 324,979 |
| 100,000 | 1,590,639 |
What happens if I breach my limit?
If your tenant exceeds its daily outbound sending limit, subsequent messages sent to external recipients will be blocked, and senders will receive one of the following bounce messages (also known as Non-Delivery Receipts or NDRs):
- Trial tenants: “Your message can’t be sent because your trial tenant has exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit)”
- Non-trial tenants: “Your message can’t be sent because your tenant exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit)”
What is the rollout schedule of these changes?
The rollout began earlier this year and is being phased in gradually:
- April 3rd, 2025: Enforcement for tenants with up to 25 licences
- April 18th, 2025: Expanded to include those with up to 200 licences
- April 28th, 2025: Expanded to include those tenants with up to 500 licences
(the rollout of these changes to those tenants with over 500 licences has been paused for the moment)
How can I monitor my external email usage?
A new Tenant Outbound External Recipients report is available in the Exchange Admin Center (EAC). This report can be used to help administrators to monitor usage and understand their current limits.
We regularly send large numbers of email to external recipients. What should I do?
Microsoft recommends using Azure Communication Services Email, which is designed for bulk and high-volume scenarios. If you’re looking to send bulk marketing emails, dedicated solutions such as Customer Insights or Click are also worth considering.
Additional Restrictions
Beyond TERRL, Microsoft may impose further restrictions based on:
- Suspicious sending behaviour
- Spam or fraudulent activity
- Late or missed payments
Microsoft’s announcement on these new limits can be viewed here and includes a useful FAQ.
Summary
This update marks a shift in Microsoft’s approach to managing outbound email traffic, with a clear focus on protecting service integrity and reducing abuse. If your organisation relies heavily on external email communications, now is the time to review your licence count, monitor usage via the EAC report, and consider alternative solutions like Azure Communication Services.
If you need help reviewing your Exchange Online setup or planning for TERRL compliance, get in touch with our team for expert guidance.