M-Daemon Server respond too slow behind the Fortigate firewall

Hello All,

I have 100D in active / passive cluster. I have a M-Daeman Server in my local environment which fetched emails from host gladiator which is hosted in USA.

I have config two policies for email. one is for outsider and other is for inside the office. Inside users are used email server private IP address to send / receive the emails from local email server whereas, local email server send / receive the emails to / from gladiator email server which is hosted in USA.

someone suggest me to change the AV profile from proxy-mode to flow-base mode I had done that but its not fruitful for me. Issue is still persist. 

What happened with us that the thousands of emails are in queues and when I restart the MDaemon services and try to resend the emails then it will send by the email server.

I have created a separate policy for email server from LAN to WAN and no security profile is enable in it and checked the status. The status is slightly better then the previous one when no profile is enable in policy.

Any suggest me? i can not restart my Mdaemon services again and again. We have send 22000 + emails on daily basis. what should I do ?

I have checked the logs and didn't find any relevant information to fix my issue. I had already opened the ticket on fortinet support portal but didn't find an appropriate solution from engineer.

Kindly advise me ASAP.