Microsoft has announced the implementation of new tenant-level outbound email limits for Exchange Online users, set to roll out in phases beginning March 2025. This change represents a shift from the previous approach that primarily relied on per-mailbox daily send limits (Recipient Rate Limit or RRL).
The new Tenant External Recipient Rate Limit (TERRL) will cap the maximum number of external recipients a tenant can send to within a 24-hour sliding window. External recipients are defined as those whose email address domains are not accepted domains in the tenant, including recipients in other Microsoft 365 tenants.
According to the announcement, the limits will be calculated based on the number of purchased email licenses a tenant has, using the formula:
500 * (Purchased Email Licenses^0.7) + 9500
For example, a tenant with 100 licenses will have a daily limit of 22,059 external recipients, while a tenant with 10,000 licenses will be capped at 324,979. Below is the limits for tenants with various license counts -
| Number of Purchased Email Licenses |
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 |
The implementation will follow a phased approach:
- March 3, 2025: Tenants with ≤ 25 email licenses
- March 10, 2025: Additional tenants with ≤ 200 licenses
- March 17, 2025: Additional tenants with ≤ 500 licenses
- March 31, 2025: All remaining tenants
Trial tenants will be limited to 5,000 external recipients per day regardless of license count. Enforcement for trial-only and single-seat tenants is already enabled.
Microsoft notes that based on service telemetry, most Exchange Online customers won't be impacted by these changes. To help administrators monitor their usage, a new report called "Tenant Outbound External Recipients" will be available in the Exchange Admin Center by late February 2025.
Understanding External Recipient
External recipients are any email addresses with domains not registered as accepted domains in your tenant, including recipients in other Microsoft 365 tenants. Each message to an external recipient counts individually against your quota, even when sending multiple messages to the same recipient.
Distribution Group messages count for each external member after expansion, and messages relayed through your tenant from on-premises environments also count.
Several message types are exempt from TERRL counting: journaling messages, automatic replies, delivery status notifications, Azure Communication Services emails, Exchange Online High-volume Email offerings, and Microsoft cloud application notifications. Messages between tenants within the same multitenant organization are treated as internal.
What is a “24-hour sliding window”?
A "24-hour sliding window" means the system continuously tracks your outbound email volume over the most recent 24 hours, not just within a fixed calendar day. The window "slides" forward in time, dropping off older messages while adding new ones to maintain a rolling 24-hour count.
The quota operates on a 24-hour sliding window rather than a fixed calendar day.
If you exceed your limit, subsequent external messages will be blocked until your volume drops below the threshold. Remember that routing through signature services or on-premises processing before delivery may result in double-counting against your quota.
For organizations needing to send more emails than their limit allows, Microsoft recommends using Azure Communication Services email for bulk or high-volume emailing to external recipients.
Certain message types will be exempt from TERRL counting, including journaling messages, automatic replies, delivery status notifications, and messages sent using Azure Communication Services email and Exchange Online High-volume Email offerings.