Sends users an email to notify of upcoming renewal in s2member

renewal_email.php

<?php

/*
 * Sends users an email to notify of upcoming renewal
 *
 */

add_action('wp', 'iart_eot_notify_set');
function iart_eot_notify_set() {
	global $current_user, $wpdb;
	$time_to_sub = 1209600; # The amount to substract from s2Member's EOT time to get notification time. 604800 = 1 week
	/**/
	if(!is_user_logged_in() || !$GLOBALS['WS_PLUGIN__']['s2member']['o']['auto_eot_system_enabled']) # If the user isn't logged in, or the Automatic EOT system is disabled...
		return; # ...we don't need to do anything
	/* Otherwise... */
	if(($eot = get_user_meta($current_user->ID, $wpdb->prefix.'s2member_auto_eot_time', true)) && !($neot = get_user_meta($current_user->ID, 's2_eot_notify_date', true))):
		$neot = $eot - $time_to_sub;
		/**/
		if($neot > time()) # If the Notification EOT time is in the future
			add_user_meta($current_user->ID, 's2_eot_notify_date', $neot); # Add the Notification EOT time
	endif;
	if($GLOBALS['WS_PLUGIN__']['s2member']['o']['auto_eot_system_enabled'] && !wp_next_scheduled('s2hack_eot_notify'))
		wp_schedule_event(current_time('timestamp'), 'daily', 's2hack_eot_notify');
}

add_action('s2hack_eot_notify', 'iart_eot_notification');
function iart_eot_notification() {
	$n_eot = time();

	if(is_array($eots = $wpdb->get_results("SELECT 'user_id' AS 'ID' FROM '".$wpdb->usermeta."' WHERE 'meta_key' = 's2_eot_notify_date' AND 'meta_value' != '' AND 'meta_value' <=". escape($n_eot)."' LIMIT 3"))):
		foreach($eots as $eot): # We need to loop through all of the IDs to send the data
			if(($user_id = $eot->ID) && is_object($user = new WP_User($user_id)) && $user->ID && !$user->has_cap('administrator')): # Set up variables
				mail_user_eot($user_id);
				delete_user_meta($user_id, 's2_eot_notify_date');
			endif;
		endforeach;
	endif;
}

function iart_mail_user_eot($user_id) {
	$user = get_user_by('id',$user_id);
	$email = $user->user_email;
	
	$headers = 'From: Tim Strawn @ IART <tim@gssalliance.com>' . "\r\n";
	
	add_filter( 'wp_mail_content_type', 'iart_html_content_type' );
	$message = '
<p>Dear '.$user->user_firstname.',</p>
<p><strong>Thank you</strong> for being a part of the IART community.<br />
Your membership will be up for renewal within 2 weeks. This means that the annual IART membership fee needs to be paid for you to maintain access to our exclusive reviews, directories, and forums.</p>
<p>Please log in and renew your membership <a href="http://gssalliance.com/renew">on the IART website</a>. If you choose not to renew your account will be demoted to <em>Consumer</em> level, where you will have access to consumer-relevant articles.</p>
<p>In 2013 we changed the software that runs the IART website, which means that we now have the ability to accept PayPal subscriptions, so following an initial renewal in our new system your membership will auto-renew in future years.<br />
You will be notified when your renewal is impending so that you have time to make any changes to your membership, if required.</p>
<p>Sincerly,<br />
Tim & the IART team</p>
	';
	remove_filter( 'wp_mail_content_type', 'iart_html_content_type' );
	
	wp_mail( $email, 'IART membership nearing renewal', $message, $headers );
}

function iart_html_content_type() {
	return 'text/html';
}

You should try using the full name of the mail function in line 33, iart_mail_user_eot