custom domain

Microsoft is advising organizations using the onmicrosoft.com domain for email to transition to a custom domain to avoid email throttling. Starting October 15, email delivery will be limited to 100 external recipients per organization within a 24-hour period, with the rollout extending to larger organizations by June 2026. This policy is in response to spammers exploiting the onmicrosoft.com domain, which affects its reputation. Organizations using a Microsoft Online Email Routing Address (MOERA) domain must migrate to a custom domain, update their primary SMTP addresses, and ensure non-test emails use the new domain. This migration may complicate administrative tasks, especially for those managing outdated products.