Reverse DNS does not match SMTP Banner

My FortiMail 200D deployed in transparent mode, when i configure SMTP proxy (System>Network>Port2 which is connected to internet) in incoming connections option I select Proxy, in this scenario an error found in mxtoolbox.com "Reverse DNS does not match SMTP Banner" so many of mails stuck on mail server. When I select Pass Through option in place of Proxy the above mentioned error solved but not showing any incoming mails log on fortimail. Kindly help me to resolve this issue.