In order for external MTAs to deliver email to the FortiMail unit, you must configure the public MX record for each protected domain to indicate that the FortiMail unit is its email server.
If your FortiMail unit will relay outgoing email, you should also configure the public reverse DNS record. The public IP address of the FortiMail unit, or the virtual IP address on a firewall or router that maps to the private IP address of the FortiMail unit, should be globally resolvable into the FortiMail unit’s FQDN. If it is not, reverse DNS lookups by external SMTP servers will fail.
For example, if the public network IP address of the FortiMail unit is 10.10.10.1, and fortimail.example.com is the FQDN of the FortiMail unit, a public DNS server’s reverse DNS zone file for the 10.10.10.0/24 subnet might contain:
1 IN PTR fortimail.example.com