There are two possible ways to remove the pop-up warning for the shared SSL certificate when using SSL/TLS for your email client.

You can setup the email using secureXXX.sgcpanel.com as mail server, where XXX should be replaced by the actual number of the server where your account is located. For example, if you are hosted on siteground164.com, you should set secure164.sgcpanel.com.

The settings should look like this:

Email Server: secureXXX.sgcpanel.com
User Name: email@yourdomain.com

The same applies for both POP3s and IMAP using SSL/TLS and also for the outgoing SMTP server.

Alternatively, you can permanently accept the shared SSL certificate for your email client. This should be possible via a check-box on the pop-up window you get.