Added better $var validation using isset to prevent 'Notice: Undefined' when WP_debug is activated.

postmarkapp.php

<?php
/*
Plugin Name: Postmarkapp Email Integrator
Plugin URI: https://wordpress.org/plugins/postmarkapp-email-integrator/
Description: Overwrites wp_mail to send emails through Postmark. This plugin is a bug fixed edition of the official Postmarkapp plugin
Author: Gagan Deep Singh
Version: 2.1
Author URI: https://gagan.pro
*/

// Define
define('POSTMARKAPP_ENDPOINT', 'http://api.postmarkapp.com/email');

// Admin Functionality
add_action('admin_menu', 'pma_admin_menu'); // Add Postmark to Settings

/*
 * Imports the settings of the official postmark plugin to this plugin.
 ***/
function pma_import_settings(){
	$options = array(
		'postmarkapp_api_key' => 'postmark_api_key',
		'postmarkapp_sender_address' => 'postmark_sender_address',
		'postmarkapp_force_html' => 'postmark_force_html',
		'postmarkapp_trackopens' => 'postmark_trackopens'
		);
	foreach($options as $here => $there){
		update_option($here, get_option($there));
	}
}

function pma_plugin_activate(){
	if(get_option('postmarkapp_api_key')===false){
		pma_import_settings();
	}
	
}
register_activation_hook( __FILE__, 'pma_plugin_activate' );

function pma_admin_menu() {
	add_options_page('Postmarkapp', 'Postmarkapp', 'manage_options', 'pma_admin', 'pma_admin_options');
}

function pma_admin_action_links($links, $file) {
    static $pma_plugin;
    if (!$pma_plugin) {
        $pma_plugin = plugin_basename(__FILE__);
    }
    if ($file == $pma_plugin) {
        $settings_link = '<a href="options-general.php?page=pma_admin">Settings</a>';
        array_unshift($links, $settings_link);
    }
    return $links;
}

add_filter('plugin_action_links', 'pma_admin_action_links', 10, 2);


function pma_admin_options() {
	if(isset($_POST['submit']) && $_POST['submit']=="Save") {
		
		if(isset($_POST['pma_enabled']) && $_POST['pma_enabled']){
			$pma_enabled = 1;
		} else{
			$pma_enabled = 0;
		}

		$api_key = $_POST['pma_api_key'];
		$sender_email = $_POST['pma_sender_address'];
		
		if(isset($_POST['pma_forcehtml']) && $_POST['pma_forcehtml']){
			$pma_forcehtml = 1;
		} else{
			$pma_forcehtml = 0;
		}
		
		if(isset($_POST['pma_poweredby']) && $_POST['pma_poweredby']){
			$pma_poweredby = 1;
		} else{
			$pma_poweredby = 0;
		}
		
		if(isset($_POST['pma_trackopens']) && $_POST['pma_trackopens']){
			$pma_trackopens = 1;
			$pma_forcehtml = 1;
		} else{
			$pma_trackopens = 0;
		}

		update_option('postmarkapp_enabled', $pma_enabled);
		update_option('postmarkapp_api_key', $api_key);
		update_option('postmarkapp_sender_address', $sender_email);
		update_option('postmarkapp_force_html', $pma_forcehtml);
		update_option('postmarkapp_trackopens', $pma_trackopens);

		$msg_updated = "Postmarkapp settings have been saved.";
	}
	?>

	<script type="text/javascript" >
	jQuery(document).ready(function($) {

		$("#test-form").submit(function(e){
			e.preventDefault();
			var $this = $(this);
			var send_to = $('#pma_test_address').val();

			$("#test-form .button-primary").val("Sending...");
			$.post(ajaxurl, {email: send_to, action:$this.attr("action")}, function(data){
				$("#test-form .button-primary").val(data);
			});
		});
		$('#pma_import_button').click(function(){
			$.post(ajaxurl, {action:'pma_import_settings'}, function(data){
				$("#test-form .button-secondary").val(data);
				if(data == 'Settings Imported'){
					location.reload();
				}
			});
		});

	});
	</script>

	<div class="wrap">

		<?php if(isset($msg_updated)): ?><div class="updated"><p><?php echo $msg_updated; ?></p></div><?php endif; ?>
		<?php if(isset($msg_error)): ?><div class="error"><p><?php echo $msg_error; ?></p></div><?php endif; ?>

		<div id="icon-tools" class="icon32"></div>
		<h2>Postmarkapp Settings</h2>
    <h3>What is Postmark?</h3>
		<p>This plugin enables WordPress blogs of any size to deliver and track WordPress notification emails reliably, with minimal setup time and zero maintenance. </p>
		<p>If you don't already have a free Postmark account, <a href="https://postmarkapp.com/sign_up">you can get one in minutes</a>. Every account comes with thousands of free sends.</p>

		<br />

		<h3>Your Postmark Settings</h3>
		<form method="post" action="options-general.php?page=pma_admin">
			<table class="form-table">
			<tbody>
				<tr>
					<th><label for="pma_enabled">Send using Postmark</label></th>
					<td><input name="pma_enabled" id="" type="checkbox" value="1"<?php if(get_option('postmarkapp_enabled') == 1): echo ' checked="checked"'; endif; ?>/> <span style="font-size:11px;">Sends emails sent using wp_mail via Postmark.</span></td>
				</tr>
				<tr>
					<th><label for="pma_api_key">Postmark API Key</label></th>
					<td><input name="pma_api_key" id="" type="text" value="<?php echo get_option('postmarkapp_api_key'); ?>" class="regular-text"/> <br/><span style="font-size:11px;">Your API key is available in the <strong>credentials</strong> screen of your Postmark server. <a href="https://postmarkapp.com/servers/">Create a new server in Postmark</a>.</span></td>
				</tr>
				<tr>
					<th><label for="pma_sender_address">Sender Email Address</label></th>
					<td><input name="pma_sender_address" id="" type="text" value="<?php echo get_option('postmarkapp_sender_address'); ?>" class="regular-text"/> <br/><span style="font-size:11px;">This email needs to be one of your <strong>verified sender signatures</strong>. <br/>It will appear as the "from" email on all outbound messages. <a href="https://postmarkapp.com/signatures">Set one up in Postmark</a>.</span></td>
				</tr>
				<tr>
					<th><label for="pma_forcehtml">Force HTML</label></th>
					<td><input name="pma_forcehtml" id="" type="checkbox" value="1"<?php if(get_option('postmarkapp_force_html') == 1): echo ' checked="checked"'; endif; ?>/> <span style="font-size:11px;">Force all emails to be sent as HTML.</span></td>
				</tr>
				<tr>
					<th><label for="pma_trackopens">Track Opens</label></th>
					<td><input name="pma_trackopens" id="" type="checkbox" value="1"<?php if(get_option('postmarkapp_trackopens') == 1): echo ' checked="checked"'; endif; ?>/> <span style="font-size:11px;">Use Postmark's Open Tracking feature to capture open events. (Forces Html option to be turned on)</span></td>
				</tr>
			</tbody>
			</table>
			<div class="submit">
				<input type="submit" name="submit" value="Save" class="button-primary" />
			</div>
		</form>

		<br />

		<h3>Test Postmark Sending</h3>
		<form method="post" id="test-form" action="pma_admin_test">
			<table class="form-table">
			<tbody>
				<tr>
					<th><label for="pma_test_address">Send a Test Email To</label></th>
					<td> <input name="pma_test_address" id="pma_test_address" type="text" value="<?php echo get_option('postmarkapp_sender_address'); ?>" class="regular-text"/></td>
				</tr>
			</tbody>
			</table>
			<div class="submit">
				<input type="submit" name="submit" value="Send Test Email" class="button-primary" />
			</div>
			<div class="submit">
				<input id="pma_import_button" type="button" value="Import Settings" class="button-secondary" />
			</div>
		</form>

	</div>

<?php
}

add_action('wp_ajax_pma_admin_test', 'pma_admin_test_ajax');
function pma_admin_test_ajax() {
	$response = pma_send_test();

	echo $response;

	die();
}

// End Admin Functionality


// Override wp_mail() if postmark enabled
if(get_option('postmarkapp_enabled') == 1){
	if (!function_exists("wp_mail")){
		function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()) {
			
			// Compact the input, apply the filters, and extract them back out
			extract( apply_filters( 'wp_mail', compact( 'to', 'subject', 'message', 'headers', 'attachments' ) ) );
			
			$recognized_headers = pma_parse_headers($headers);
			
			if(isset($recognized_headers['Content-Type']) && stripos($recognized_headers['Content-Type'],'text/html')!==false){
				$current_email_type = 'HTML';
			}
			else{
				$current_email_type = 'PLAINTEXT';
			}
			
			// Define Headers
			$postmark_headers = array(
				'Accept' => 'application/json',
				'Content-Type' => 'application/json',
		        'X-Postmark-Server-Token' => get_option('postmarkapp_api_key')
			);

			// Send Email
			if(is_array($to)){
				$recipients = implode(",", $to);
			} else {
				$recipients = $to;
			}

			//@todo need to add a count for receipients(including Cc and Bcc so that they may not go beyond 20
			
			// Construct Message
			$email = array();
			$email['To'] = $recipients;
			$email['From'] = get_option('postmarkapp_sender_address');
			$email['Subject'] = $subject;
			$email['TextBody'] = $message;
			
			if(isset($recognized_headers['Cc']) && !empty($recognized_headers['Cc'])){
				$email['Cc'] = $recognized_headers['Cc'];
			}
			
			if(isset($recognized_headers['Bcc']) && !empty($recognized_headers['Bcc'])){
				$email['Bcc'] = $recognized_headers['Bcc'];
			}
			
			if(isset($recognized_headers['Reply-To']) && !empty($recognized_headers['Reply-To'])){
				$email['ReplyTo'] = $recognized_headers['Reply-To'];
			}

			if($current_email_type == 'HTML'){
				$email['HtmlBody'] = $message;
			}
			else if(get_option('postmarkapp_force_html') == 1 || get_option('postmarkapp_trackopens') == 1){
				$email['HtmlBody'] = pma_convert_plaintext_to_html($message);
			}

			if(get_option('postmarkapp_trackopens') == 1){
				$email['TrackOpens'] = "true";
			}

			$response = pma_send_mail($postmark_headers, $email);
			
			if(is_wp_error($response)){
				return false;
			}
			return true;
		}
	}
}

function pma_convert_plaintext_to_html($message){
	$message = nl2br( htmlspecialchars( $message ));
	return $message;
}

/*
 * Parses the $headers string or array and create a recognizable headers array
 */
function pma_parse_headers($headers){
	if(!is_array($headers)){
		if(stripos($headers, "\r\n")!==false){
			$headers = explode("\r\n", $headers);
		}
		else{
			$headers = explode("\n", $headers);
		}
	}
	$recognized_headers = array();
	$headers_list = array(
		'Content-Type',
		'Bcc',
		'Cc',
		'Reply-To'
	);
	if(!empty($headers)){
		foreach($headers as $key=>$header){
			//@todo there are three kind of people, one who send headers as 
			//string, second who send headers as associative array, third who 
			//send headers as simple text array
			$header = explode(':', $header);
			foreach($headers_list as $header_name){
				if(stripos($header[0], $header_name)!==false){
					$recognized_headers[$header_name] = trim($header[1]);
					unset($headers[$key]);
					break;
				}
			}
		}
	}
	return $recognized_headers;
}

function pma_send_test(){
	$email_address = $_POST['email'];

	// Define Headers
	$postmark_headers = array(
		'Accept' => 'application/json',
		'Content-Type' => 'application/json',
        'X-Postmark-Server-Token' => get_option('postmarkapp_api_key')
	);

	$message = 'This is a test email sent via Postmark from '.get_bloginfo('name').'.';
	$html_message = 'This is a test email sent via <strong>Postmark</strong> from '.get_bloginfo('name').'.';
	
	$email = array();
	$email['To'] = $email_address;
	$email['From'] = get_option('postmarkapp_sender_address');
    $email['Subject'] = get_bloginfo('name').' Postmark Test';
    $email['TextBody'] = $message;
    
    if(get_option('postmarkapp_force_html') == 1){
    	$email['HtmlBody'] = $html_message;
	}

	if(get_option('postmarkapp_trackopens') == 1){
		$email['TrackOpens'] = "true";
	}

    $response = pma_send_mail($postmark_headers, $email);
	
	if (is_wp_error($response)){
    	return 'Test Failed with Error "'.$response->get_error_message().'"';
    } else {
    	return "Test Sent";
   	}

    die();
}


function pma_send_mail($headers, $email){
	$args = array(
		'headers' => $headers,
		'body' => json_encode($email)
	);
	do_action('before_wp_mail');
	
	$response = wp_remote_post(POSTMARKAPP_ENDPOINT, apply_filters('pma_mail_args',$args));

	do_action('after_wp_mail');
	
	if(is_wp_error($response)){
		return new WP_Error('CONNECTION_TIMEOUT','Connection Timeout');
	}
	else if(isset($response['response']['code'])){
		if($response['response']['code'] == 200) {
			return true;
		} else {
			$failure_message = '';
			if(isset($response['body'])){
				$error = json_decode($response['body'],true);
				if(isset($error['ErrorCode'])){
					$error_code = $error['ErrorCode'];
				}
				else{
					$error_code = '000';
				}
				if(isset($error['Message'])){
					$error_message = $error['Message'];
				}
				else{
					$error_message = 'Unknown Error';
				}
				return new WP_Error($error_code,$error_message);
			}
			
		}
	}
	else{
		return new WP_Error('NO_RESPONSE','No Response from the PostMark Server');
	}
	wp_mail();
	
	
}

/*
 * Changes the default http request timeout of wordpress from 5 seconds to 60
 * seconds so that the request the postmark api can be successfully executed.
 */
function pma_filter_http_request_timeout($current_timeout) {
	if (intval($current_timeout) < 60){
		return intval(60);
	}
	else{
		return intval($current_timeout);
	}
}

/*
 * Adds timeout filter so that mail function can get enough time to contact the
 * postmark api servers
 */
function pma_add_timeout_filter(){
	add_filter( 'http_request_timeout', 'pma_filter_http_request_timeout');
}
add_action('before_wp_mail','pma_add_timeout_filter');

/*
 * Removes the timeout filter after the mail function has been successfully 
 * executed
 */
function pma_remove_timeout_filter(){
	remove_filter( 'http_request_timeout', 'pma_filter_http_request_timeout');
}
add_action('after_wp_mail','pma_remove_timeout_filter');

function pma_admin_import_settings(){
	pma_import_settings();
	echo "Settings Imported";
	die();
}
add_action('wp_ajax_pma_import_settings', 'pma_admin_import_settings');
?>