About SSL error

Hi 

 i am using 6525e NGFW Huawei.

i got 10 users behind Huawei Fw.

Some users lock their computers while adding an attachment file to to the mail.

Our mail server is at intranet. 

Traffic like this: User terminal --> Huawei FW --> Mail server.

i checked event log on user terminal.

I am getting multiple Schannel errors every 5mins or so.  The error is a Schannel error with Even ID 36882 and reads:The certificate received from the remote server was issued by an untrusted certificate authority. Because of this, none of the data contained in the certificate can be validated. The TLS connection request has failed. The attached data contains the server certificate.

Then I added a detection policy to mail.xx.com.tr in the form of "Do not decrypt". i added additional

Is there a log where I can see the ssl/tls conflicts?

Even if I see this, will it be enough to solve the problem?

but still users get the same error from every 5 mins.

Then I added the ssl domain name in the white list. But I still get the error from time to time.

i canceled the decryption while going to my own servers.But we still get the error

Where is wrong  ?

Could you please help me ?

Regards