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 Management Tools -> Add / Remove E-mail account(s).
        From "Account @ Domain", select email you want to set.
        At "Action" of the Email above, click More -> Configure Email Client.

Then use the following values to place in Step 3 -> System -> Email System -> Type of Mail Service: select SMTP
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Outgoing Server: {related to the email in number 2}
port:
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        3) Sendmail:

        Sendmail path: {default sendmail path: /usr/sbin/sendmail}

Sendmail argument: {default argument: /usr/sbin/sendmail}