Replacement Messages with DLP

Hello experts,

I'm fairly new to FortiGate and FortiOS....

we configured the test DLP sensors which is working great and block anything matches the filters. also we enabled the ssl/ssh deep inspection and all is working fine.

now, we are trying to configure the replacement messages which are working as expected for some protocols like FTP, HTTPS and user can see the replacement message when trying to send something that is blocked. but when trying to test the same for gmail in browser, we can't see any replacement message only some error like below

"Error Oops... a server error occurred and your email was not sent. (#403)"

is this something can be configured/change with a replacement message? 

Thanks,

Nemer