Receiving email form some domains problem after setup fortigate

[CAD]

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

[emnoc]

You need to provide more but that seems like delay and maybe  greylisting. Since you have a 4XX message that means something has responded which is better than no message.

Can draft a drawing of the flow of traffic?

[CAD]

thanks for quick reply,

the traffic : internet (ISP)-------- fortigate ---------------coreswitch 

please check attached file

thanks

[CAD]

any Update ?

[emnoc]

yes , greylisting this normal and not avoidable. Does email domains that delay you finally allow you to send?

[CAD]

thanks for reply ,

I really did not understand, please explain to me more.

thanks.

[CAD]

some one advise me to run below command:

set tcp-mss-sender1452 set tcp-mss-receiver1452

please advise me ?

thanks

[CAD]

I have doubt that the IPS  service it is  working to drop traffic?

Thanks