singed pdf files get blocked when user is sending from shared mailbox

Hi, in exchange environment, I have my email account and also I have shared mailbox linked.
I can sent the singed pdf file attached to email from my email but when I send it from shared mailbox, I get the following message everytime:

"From: email address removed for privacy reasons <email address removed for privacy reasons> Subject: Executable File Violation You attempted to send a message that contained an executable file. Our company policy prohibits the sending of executable files via email. The message was not delivered."

We have symantec spam filter in organization, which I think is causing that error, or it can be exchange policy which is more restrictive (on default or someone configured it so),

I wonder if it is possible to address this issue and let the singed pdf flow from shared mailbox.

What can I do to fix it?

From where should I fix it - exchange management / shared mailbox policies or where do I find that options to configure correctly?

Thanks

George