Outlook for Mac add-in, and for a particular user, we are getting prompts

We have an Outlook for Mac add-in, and for a particular user, we are getting prompts that a secure connection cannot be established with the server and that the user needs to trust the certificate. If he trusts the certificate, the add-in will work, but we want to bypass that step and avoid any prompts, Do you have any idea why the prompt might be coming and how to prevent it?