An inbound connector is a mail-flow trust rule in Microsoft 365 and Exchange Online that tells the service which external systems are allowed to send mail into the tenant. It usually identifies approved sources by IP address, certificate, or other explicit transport conditions. In practice, the connector helps Microsoft decide whether a message should be treated as trusted relay traffic rather than anonymous internet mail.
This matters because email security often depends on where a message came from, not just what it says. Defenders use inbound connectors to support legacy devices, third-party gateways, and partner systems without opening the tenant to everyone. Attackers look for mistakes here: overly broad IP ranges, weak certificate checks, or mail paths that bypass the expected connector can let internal-looking messages reach users. In phishing defenses, reviewing inbound connector rules is a key way to limit impersonation and reduce trust in unauthenticated mail.