You can set Contact Form Sender for Online Form, Contact Us in Step 3 -> System -> Email System
At Type of Mail Service: {mail() function, SMTP, Sendmail}

       1) mail() function: Simple email sender from your email address.

2) SMTP: The Type of Mail Service: SMTP, will need to set as the same of email client config from cPanel.
Please check as followings:
1. Check and make sure the Firewall does allow TCP_OUT port 25.
2. Check the email client config in cPanel -> Email Accounts
        Select email address you want to use.
        Click "Connect Devices" of the Email above, click Setup Mail Client.

And use values here to place in Step 3 -> System -> Email System -> Type of Mail Service: select SMTP

For Secure SSL/TLS Settings:
SMTP host: ssl://mail.{your-domain}
SMTP port: default 465


For Non-SSL Settings:
SMTP host: mail.{your-domain}
SMTP port: default 25

       3) Sendmail:
        Sendmail path: /usr/sbin/sendmail (default)
        Sendmail argument: -t -i (default)