The error message you've quoted pretty much tells you exactly what the problem is.
The SSL certificate on your OWA server does not match the hostname of the server. One example of why this might occur is if you use a different DNS namespace internally and externally. e.g.
exchange.example.local internally and
exchange.example.com externally and you are attempting to connect from an external location, when the certificate was designed for internal only use. If you don't use a certificate that includes the other (external in my example) hostname as a Subject Alternate Name (SAN), then you will have this problem.
Unfortunately, as @Shane has pointed out, you don't give enough info for anyone to give any specific help to you, however if you could edit your question and give a little more info, I'm sure you'll get a more specific answer than is currently possible.