There is a Failure sending e-mail due to the email protocol not matching when attempting to reach out to the email server. The Email Server settings are incorrect or the Cb Response server is unable to reach the email server
Verify that you have the smtp connection type set correctly under Administration > Settings > E-Mail.
Verify Perimeter devices (firewall, proxy etc) allow pass-through SSL traffic. If you would like to troubleshoot the E-mail server connection, we'd recommend bringing this issue up with your networking team or email server team. You could start out with ping, tracert and a wireshark capture
If you have no policies against it, another viable option would be to just the Carbon Black External Mail Server option instead. This setting can be changed in the UI under Administration > Settings > E-Mail to Use Carbon Black External Mail Server (Secure HTTPS POST to api.alliance.carbonblack.com) and save changes