For changing the mail hosts name from your local machine name to domain name.Setting the hostname in sendmail.

setting_hostname_sendmail_in_ubuntu.md

If you need to change the hostname that Sendmail announces itself as, just add the following to sendmail.mc:

define(`confDOMAIN_NAME', `mail.yourdomain.com')dnl

For example-: Your domain name is www.xyz.com, then

define(`confDOMAIN_NAME', `www.xyz.com')dnl