Office 365 - Exchange Error

Hi,

I am using an Apple Mac - with Outlook 2016 and getting the following error when I log into Outlook.

Verify Certificate

A secure connection cannot be established with the server xxxx.xx.xx because its intermediate or root certificate cannot be found.
Do you want to continue.

If you continue, the information that you view and send will be encrypted, but will not be secure.

There is an option to always trust. But this does not seem the best way of fix it.

So how can I resolve this issue so the dialog does not appear.

Thanks,

Ward.