This plugin does not take into account that wp_mail() $headers variable can be an array. Many plugins, such as Gravity Forms, supply the $headers information as an array. It’s a very quick fix if you’d like to get this up and running again, while waiting for the plugin developer to update this. In the postmark.php file, on line 167, just below the line that says:

function wp_mail( $to, $subject, $message, $headers = '', $attachments = array()) {

add the following to a new line:

if( is_array($headers) ) $headers = implode("\r\n", $headers);

https://www.ads-software.com/plugins/easy-postmark-integration/