Firewall Rule vs. Email Policy

Hello Folks,

i'm a bit confused about the whole mail scanning configuration. If you create a new SMTP policy under Email -> Policies & Exeptions you can set Malware Protection settings like Scan Engines and antivirus action. If you create a firewall rule, you can set email content scanning for e. g. SMTP. Do i have to set both? I thought that the MTA will scan based on the policy i mentioned first. When do i havew to set the scanning via firewall rule?

Thanks in advance!