The problem is this:
The SMTP you are using provides a DH key that is too small. Are you using your own SMTP server? If so, you are in good company. Rackspace had the same problem up until recently.
Email SSL Errors after Update to 2.4.0.beta4 - #11 by gerhard contains instructions on how to debug the issue and a temporary workaround. But you or your provider should really increase the size of the DH key.