An accepted domain is a domain name that Microsoft 365 or Exchange Online treats as belonging to a tenant for mail-flow decisions. If a message claims to use that domain, the platform can apply tenant-specific routing, anti-spoofing, and transport rules instead of treating it as arbitrary external mail.
This matters because attackers often try to make messages look internal. In edge cases around Direct Send or anonymous relay, a message that references an accepted domain can appear more trustworthy to both the mail system and the recipient. Defenders use accepted domains to define where their email namespace begins and ends, then combine that with connectors, authentication, and sender validation. Reviewing accepted domains helps spot unwanted internal-looking mail paths and reduces the chance that unauthenticated messages can impersonate the organization’s own identity.