Receiving email form some domains problem after setup fortigate

Hello,

we have FortiGate 200D and firmware 5.2.3 we have prepared FortiGate firewall recently. We are facing a problem with some of the domains are not able to receive e-mail messages from the domains. I have looked at records on exchange server file and found the error below:

"451 4.7.0 Timeout waiting for client input"

Before firewall preparation and we are able to receive e-mail messages are usually from this domains as mentioned above.

Thanks