Change "FROM" mailadress in a function - php

Question

I installed a plugin called Message 2 Auther and its doing the job that i request. But, i dont want the "FROM" adress to be the Wordpress default mail, i want it to be from the user that wrote the message.
I've changed so that the subject is what i want it to be, but i dont know where to find the default mail.
Here is the code:
<?php defined('ABSPATH') or die('No script kiddies please!'); ?>
<?php
function m2a_getMessageBoxHTML() {
$messagebox = '<script src="https://www.google.com/recaptcha/api.js">
</script><form class="form" method="post" action="' .
esc_url(admin_url('admin-post.php')) . '"><input style="margin-bottom:15px;"
type="text" name="subject" placeholder="Subject">';
if (!is_user_logged_in()) {
$messagebox .= '<input style="margin-bottom:15px;" type="email"
name="user_email" placeholder="Email">';
}
$messagebox .= '<textarea style="margin-bottom:15px;" name="message"
placeholder="Message"></textarea>
<input type="hidden" name="action" value="m2a_new_message"
/>
<input type="hidden" name="post_id" value="' . get_the_ID()
. '" />';
$options = get_option('m2a_setting');
if ($options['googlecaptcha'] && $options['googlecaptchapublickey']) {
$messagebox .= '<div class="g-recaptcha" data-sitekey="' .
$options['googlecaptchapublickey'] . '"></div>';
}
$messagebox .= '<input type="submit" class="button btn"
name="submit_message" value="submit" />
</form>';
return $messagebox;
}
function m2a_getPopupHTML() {
add_thickbox();
$messagebox = '<div id = "my-content-id" style = "display:none;">
<form class = "form" method = "post" action = "' . esc_url(admin_url('admin-
post.php')) . '" style = "text-align:center;">';
if (!is_user_logged_in()) {
$messagebox .= '<p class="mail_rubrik">Mail:</p><input
class="ruta_rubrik" style = "margin-bottom:15px;" type = "email" name =
"user_email" placeholder = ""><br/><br/>';
}
$messagebox .= '<p class="meddelande_text">Meddelande:</p><textarea
class="meddelande_ruta" style = "margin-bottom:15px;" name = "message"
placeholder = "" rows = "5"></textarea>
<input type = "hidden" name = "action" value = "m2a_new_message" />
<input type = "hidden" name = "post_id" value = "' . get_the_ID() . '" />';
$options = get_option('m2a_setting');
if ($options['googlecaptcha']) {
$messagebox .= '<div class="g-recaptcha" data-sitekey="' .
$options['googlecaptchapublickey'] . '"></div>';
}
$messagebox .= '<br/><br/><input type = "submit" class = "button btn" name =
"submit_message" value = "Skicka" />
</form></div>
<a href = "#TB_inline?width=auto&height=auto&inlineId=my-content-id" class =
"thickbox btn button">Skicka ett meddelande</a>';
return $messagebox;
}
function m2a_sendemail($to, $subject, $message, $usermail = 0, $post_id) {
$post_title = get_the_title($post_id);
$site_name = get_bloginfo('url');
if ($type == 'author') {
$message = "Du har ett nytt meddelande från {$usermail}<br/>Meddelande:
{$message} </a>";
} else {
$message = "Du har ett nytt meddelande från {$usermail}<br/>Meddelande:
{$message}<br/></a>";
}
$subject = "Nytt meddelande på 4 Happy Pets";
wp_mail($to, $subject, $message, array('Content-Type: text/html;
charset=UTF-8'));
}
function m2a_aftercontent() {
$m2a_setting = get_option('m2a_setting');
if (isset($m2a_setting['aftercontent']) && $m2a_setting['aftercontent'] ==
1) {
if ((!isset($m2a_setting['nonuser'])) || ($m2a_setting['nonuser'] == 1
&& is_user_logged_in())) {
function m2a_messagebox($content) {
$m2a_setting = get_option('m2a_setting');
if (is_single()) {
if ($m2a_setting['showas'] == 'messagebox')
return $content . m2a_getMessageBoxHTML();
else
return $content . m2a_getPopupHTML();
}
}
add_filter('the_content', 'm2a_messagebox');
}
}
}
add_action('init', 'm2a_aftercontent');
/*
* Saving Data to database
*/
function m2a_message_db_store() {
global $wpdb;
// global $post;
$postid = $_REQUEST['post_id'];
$authorid = get_post_field('post_author', $postid);
$subject = $_REQUEST['subject'];
$message = $_REQUEST['message'];
$options = get_option('m2a_setting');
if ($options['googlecaptcha']) {
$captcha = $_REQUEST['g-recaptcha-response'];
$response = json_decode(file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=".$options['googlecaptchasecretkey']."&response=".$captcha."&remoteip=".$_SERVER['REMOTE_ADDR']), true);
if ($response['success'] == false) {
wp_safe_redirect(wp_get_referer());
}
}
if (!is_user_logged_in()) {
$userid = $_REQUEST['user_email'];
$usermail = $userid;
} else {
$userid = get_current_user_id();
$usermail = get_userdata($userid)->user_email;
}
$tableName = $wpdb->prefix . 'm2a_message';
$wpdb->insert($tableName, array('user_id' => $userid, 'author_id' =>
$authorid, 'post_id' => $postid, 'subject' => $subject, 'message' =>
$message));
$options = get_option('m2a_setting');
if (isset($options['emailtoauthor']) && $options['emailtoauthor'] == 1)
{
$to = get_userdata($authorid)->user_email;
m2a_sendemail($to, $subject, $message, $usermail, $postid);
}
if (isset($options['emailtouser']) && $options['emailtouser'] == 1) {
m2a_sendemail($usermail, $subject, $message, 0, $postid);
}
wp_safe_redirect(wp_get_referer());
}
add_action('admin_post_nopriv_m2a_new_message', 'm2a_message_db_store');
add_action('admin_post_m2a_new_message', 'm2a_message_db_store');
// Create shortcode
function messagebox($atts = array()) {
$a = get_option('m2a_setting');
$atts = shortcode_atts(array(
'style' => 'default',
), $atts, 'message2author');
if ((!isset($a['nonuser'])) || ($a['nonuser'] == 1 &&
is_user_logged_in())) {
if ($atts['style'] == 'messagebox') {
return m2a_getMessageBoxHTML();
} elseif ($atts['style'] == 'popup') {
return m2a_getPopupHTML();
} elseif ($atts['style'] == 'default') {
if ($a['showas'] == 'messagebox') {
return m2a_getMessageBoxHTML();
} elseif ($a['showas'] == 'popup') {
return m2a_getPopupHTML();
}
}
}
}
add_shortcode('message2author', 'messagebox');
?>