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Title
Domain Rewrite and Tenant External Recipient Rate Limit (TERRL) -
Description
How does Microsoft Exchange Online Tenant Outbound Email Limits “Tenant External Recipient Rate Limit or TERRL” relate to Domain Rewrite Projects?
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Resolution
Tenant External Recipient Rate Limit (TERRL) is a relatively new threshold introduced by Microsoft Exchange Online. Any recipient with a domain that is not an accepted domain in your tenant is considered an external recipient and when a message is sent to them it will count against your quota. This includes messages sent to recipients in other Microsoft 365 tenants including On-Prem Exchange Servers or Partner Mail Services.
In the case of Domain Rewrite, both outbound and inbound email messages are routed from the customer’s tenant to the rewrite service for address rewrite and then routed back to the tenant for final delivery. As a result, each message processed through Domain Rewrite is counted as two external messages by Exchange Online. Because of this behavior, both the source and target tenants must have sufficient license capacity to avoid exceeding Microsoft’s TERRL limits. For details on the TERRL limits, please refer to the below table as described by Microsoft Introducing Exchange Online Tenant Outbound Email Limits | Microsoft Community Hub.
Sample limits for tenants with various license counts are shown below:
Number of Non-trial Email Licenses Tenant External
Recipient Rate Limit1 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 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: 550 5.7.232 - 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: 550 5.7.233 - Your message can't be sent because your tenant exceeded its daily limit for sending email to external recipients (tenant external recipient rate limit)
Additional details about Tenant External Recipient Rate Limit (TERRL) can be found at Introducing Exchange Online Tenant Outbound Email Limits | Microsoft Community Hub.
To best ensure that the tenants have sufficient license capacity, ensure that you follow the best practices below:
- Source rewrite as Target: Although Domain Rewrite works with target accounts that are MailUsers, Guests, Mailbox Enabled Users, the recommendation is to always mailbox-enable the target accounts.
- Target rewrite as Source: Although Domain Rewrite works with source user and shared mailboxes, the recommendation is to keep source user mailboxes licensed instead of converting them to shared.