FortiSandbox Cloud - unable send/forward some attachments

Hello,

I want to consult the settings of my antivirus scan. On our FGT we have a Policy for checking email communication with our email. server. It's Deep Inspection, clients have FGT certificates signed by our AD. Everything works perfectly. However, I would also like to start using the FortiSandbox Cloud (when it's free). However, when I turn on suspicious files in the cloud in the AV profile, users are unable to send / forward some attachments - sometimes it's .pdf, sometimes .docx, etc. It's always random. I did not see any connection with certain attachments, etc. As an email server we use IceWarp and there is always an error message "INCOPLETE SESSION" in the server log. I tried to disable SSL inspection, but it behaves the same way. Does any of you use FortiSandbox cloud for this purpose? If so, please give advice on the settings.

Thank you.

Jirka