Microsoft 365 Tenants Need Vanity Domains to Send External Email

Microsoft will impose a throttling limit for external recipients for tenants that use MOERA domain addresses to send outbound email. The limit is designed to stop tenants using mailboxes with primary SMTP addresses from MOERA domains from sending email, a technique that’s often used by spammers. This shouldn’t cause a problem for legitimate organizations who already have vanity domains, but it might stop some spam.

https://office365itpros.com/2025/08/25/moera-domain-limit/