Mail Server Names to Customer Domain mail.domain.tld

I noticed one of domain (main and only domain under a cPanel account) uses cPanel hostname instead of their domail mail.domain.tld, for SSL/TLS access. My WHM and cPanel host name is like cp.myservicedomain.tld and this is shows under "connect devices" option for that cPanel account. I check other accounts and they use their own domain names. All sites were issued AutoSSL Where or how do I force this account to use their domain mail.domain.tld ?