"As of October 15, Microsoft has announced that it will begin throttling emails. The limit will be set to 100 external recipients per organization per 24-hour rolling window. From December 1, Microsoft will start rolling out the restrictions across tenants, starting with tenants with fewer than three seats and eventually reaching tenants with more than 10,001 seats by June 2026."
"The problem the WIndows maker is trying to deal with is spammers exploiting a newly created tenant and sending out bursts of spam email before the company can intervene. This activity means that the onmicrosoft.com domain can be flagged as suspect or, as Microsoft puts it, "degrades this shared domain's reputation." The domain onmicrosoft.com (and others like it, such as onmicrosoft.de) is automatically provided when an organization creates a new Microsoft 365 tenant."
Microsoft will impose email throttling for tenants using onmicrosoft.com addresses, capping delivery to 100 external recipients per organization in any 24-hour rolling window. The restrictions begin October 15 and will roll out from December 1 through June 2026, starting with very small tenants and progressing to very large tenants. The action targets spam runs from newly created tenants that damage the shared onmicrosoft.com domain reputation. Organizations must acquire and use custom domains for non-test email, change tenant default domains away from MOERA addresses, and update mailbox primary SMTP addresses to the custom domain, which may cause operational challenges.
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