What happens when an email is rejected by an intermediary filter?

Intermediary filter rejection happens at gateway level:

What intermediaries are:

Security gateways (Proofpoint, Mimecast, Barracuda). Cloud email security services. Corporate email filters. Anti-spam appliances.

How rejection occurs:

Your message reaches the gateway. Gateway evaluates against its rules. Rejection happens before reaching mailbox. Bounce indicates gateway, not destination server.

Bounce characteristics:

May identify gateway in response. Different codes than final destination would use. May be stricter than native ISP filtering.

Challenges:

Gateway rules may differ from destination. Hard to know all gateways in use. Corporate security policies vary widely.

Resolution:

Identify gateway from bounce. Check gateway-specific documentation. May need to contact recipient's IT. Enterprise whitelisting may be required.

Intermediaries are the outer harbor defenses. You may be blocked before reaching the inner port.