FortiMail has two transparent proxies: an incoming proxy and an outgoing proxy. The proxies’ degree of transparency at the IP layer and at the SMTP layer varies by your configuration. Proxy behaviors are configured separately based on whether the SMTP connection is considered to be incoming or outgoing. Depending on your configuration, a FortiMail unit operating in transparent mode may implicitly use its built-in MTA instead.

Depending on your network topology, verify that email is not being scanned twice.