FortiSandbox as an alternative solution to FortiMail?

Hi

not at all with BCC mode.

BCC Mode is only for detection, if the email is infected, local email users could be infected.

Any action to take after detection is manual o through API

With MTA adapter, FSBox receive emails, scan it and relay these mails if they're are clean, or quarantine if infected and quarantine is enabled, or merely tags it if not.

MTA adapter is an addon licensed feature.

FortiMail otherwise a a complete email security solution,  you can also license fortisandboxing cloud service for it.