Microsoft is in the middle of one of the most consequential changes to outbound email in Exchange Online this year. According to message center announcement MC1454397, published on 14 August 2026 and updated on 29 September, the company is recalculating the Tenant External Recipient Rate Limit, known as TERRL, for newly created tenants, trial tenants and organizations using free Education licenses. The rollout began in mid-September 2026 and is expected to complete by the end of November 2026 for worldwide tenants.
The TERRL is not a per mailbox rule. It caps how many external recipients an entire tenant can email within a rolling 24 hour window, and every mailbox and application sending through the tenant draws from the same shared quota. Microsoft frames the change as a way to strengthen protection against abuse and ensure fair use of Exchange Online resources, and the pattern it targets is easy to guess: freshly minted tenants blasting large volumes of outbound mail, a signature behavior of spam operations and business email compromise actors.
The numbers Microsoft published
The announcement is unusually specific about the new quotas. A tenant younger than 31 days gets only 10 percent of its standard calculated quota. Between 31 and 60 days old it gets 25 percent. Only after 60 days does a tenant receive its full quota. For tenants without free EDU licenses the standard formula stays 500 multiplied by the number of purchased email licenses raised to the power of 0.7, plus 9,500. That means a brand new tenant with ten licenses can reach barely a thousand external recipients a day in its first month instead of roughly twelve thousand.
Trial tenants take the sharpest cut. Their daily TERRL quota drops from 5,000 external recipients to 500, a tenfold reduction that effectively ends any idea of running a serious campaign from a trial subscription. Educational organizations with free EDU licenses such as A1 get their own formula, 500 multiplied by purchased email licenses raised to the power of 0.7 plus free EDU licenses raised to the power of 0.3, plus 9,500, so free licenses still count toward the quota but at a much lower weighting than paid ones.
When a tenant goes over its quota, outbound messages to external recipients are simply blocked until usage falls below the rolling limit. Senders see a non-delivery report that identifies the rule that stopped them: 550 5.7.232 for trial tenants and 550 5.7.233 for everyone else.
Why a sending limit is a deliverability story
For anyone who runs email marketing or manages a sending domain, this change deserves a place next to SPF, DKIM and DMARC in your planning, because it institutionalizes something mailbox providers have long demanded: new senders must earn volume gradually. A throttled young tenant cannot blast a cold list of 50,000 contacts on day one no matter how clean its authentication is. The quota curve is a forced warm-up period at the platform level.
That is good news for legitimate senders. Volume spikes from brand new infrastructure are one of the strongest spam signals in every reputation system, and the senders who scale deliberately, watch engagement and keep complaint rates low are the ones who reach the inbox once the quota opens up. The change also closes a convenient lane for attackers, because a spun-up tenant with stolen licenses was previously a cheap way to relaunch a phishing or BEC campaign at scale.
Microsoft itself recommends reviewing the Tenant Outbound External Recipients Rate report in the Exchange admin center, under Reports, Mail flow. The report shows the current quota, the outbound recipient volume, the percentage of quota consumed, the number of blocked recipients and whether enforcement is enabled. For tenants that regularly operate near or above quota, Microsoft suggests moving high-volume, application-driven email to a third-party bulk email service or buying more licenses to cover the volume.
What to do before the end of November
Start by finding out whether any of your sending infrastructure depends on a tenant that falls into the affected categories. A new subsidiary tenant, a recently created environment for a product launch or a school license footprint all qualify. Check the EAC report now rather than waiting for a 550 5.7.233 bounce to tell you something is wrong.
Plan your volume around the quota curve. If a new tenant will carry important communications in its first 60 days, either budget the sends against the reduced percentages or shift them to a dedicated email platform where the limit does not apply. And whatever infrastructure you send from, make sure SPF, DKIM and DMARC alignment are verified from the start, because a tenant that both throttles volume and authenticates cleanly is the best possible neighbor in the inbox.
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