When you try to access a service such as EMAIL on your server over the SSL (Secure Socket Layer) protocol, the server has to identify itself with an SSL certificate to your web-browser. In order for web-browsers to trust the certificate that the server has presented, the SSL certificate must be issued by a valid Certificate Authority (CA). By default the SSL certificate that would be configured on your server would be a self-signed one, meaning that it has not been issued by a CA, but instead your own server has signed the certificate as being valid.
This works fine for encrypting data, but it will present you with an error or warning in your web-browser when trying to access the secure content.
This warning is simply letting you know that the SSL certificate was self-signed. In the case of accessing your own server this isn't a problem at all, and you can simply tell your web-browser to accept the self-signed SSL certificate and continue. Where you would typically take caution on these types of errors would be if you were accessing your bank or a credit card's website, as that could be an indication your secure data isn't properly going to the right server.
In Safari:
1. QUIT mail and then re-open it.
2. When prompted with the certificate warning, simply click SHOW CERTIFICATE. You may need to attempt to send/receive mail first.
3. Click ALWAYS TRUST box.
4. Click CONTINUE.
