I cannot send email through my Outlook for Mac 2011 account because it says that the correct root certificate is not installed. How do I fix this?

I have had this program for a few years and have never had this problem before.  In fact, I was using the account yesterday.  The entire wording is "Unable to establish a secure connection to the server because the root certificate is not installed"  Any help would be appreciated as this is the account I use for my business