This page documents the FROM_USER configuration setting in FormMail.
Type Of Setting
FROM_USER is an OPTIONAL setting, which means...
OPTIONAL : you can change this setting if you need to, but the default value is fine for most circumstances.
Sender for alerts and auto responses.
Set FROM_USER to the email address that will be the sender of alert/error messages and Auto Responses (if you're using the auto responder feature).
If you do not specify FROM_USER, formmail.php uses "FormMail@SERVER" where SERVER is determined from your web server. If you set it to "NONE", then no sender is specified.
NOTE: FROM_USER is not used for form results, it is only used for alert messages and Auto Responses. If you want to set the From line in the emails you receive for form submissions (for example, to your own email address) this is not the correct setting. Use the FromAddr feature in mail_options.
For detailed information about how FormMail specifies the From address and Sender address of emails, view the detailed document about From Address and Sender.
$FROM_USER = "";
$FROM_USER = "firstname.lastname@example.org"; // example - replace with your email address
$FROM_USER = "NONE"; // use this to show no sender