Every Time I open Outlook I get the following error message

"The server You are connecting to is using a security certificate that could not be verified.

A Certificate chain processesed correctly, but terminated in a root certificate which is not trusted by the Trust Provider.

Do You want to continue using this server"

Although I can still use everything in Outlook I would like to know why I am getting this message and how to get rid of the message.

The server is of course a Linux Server.

Could Somebody please help me with this error, if possible, Thank you