cPanel is using SSL with the domain name as the mail server. You set the POP3 port to 995.
You may use cp.providersite.net to get SSL instead.
Customers can now use their domain name and automatically get SSL.
All customers can (and should!) now use "mail.your-domain-name.com". However, if the DNS has not yet been switched to Hostica.net, the AUTOSSL script is unable to verify the domain and install the SSL certificate. Once the domain is switched the DNS it will be about 24 hours until SSL certs are installed for all the subdomains. (Including webmail.domain.com)