Webmail on Internet Explorer, Blackberry etc.
The server your webmail resides on uses a self-signed certificate and is very secure in that sense.
Unfortunately, some browsers e.g. recent versions of IE on Windows, will not allow you to approve self-signed certificates.
Internet Explorer/Windows can be modified to allow self-signed certs, but t is not an easy fix, so would need to be carried out by your tech support or by Googling for details. You make these changes at your own risk.
The easiest fix for desktop and mobile devices is to install a better browser e.g. Firefox or use a mail app that connects to SMTP or IMAP mail on the server.