Fatal error when trying to send a bounce testmail

When I have set my mail information and try to send a bounce testmail from the plugin I get this message:

<b>Fatal error</b>: Call to undefined function imap_open() in <b>/home/hulstein/domains/hulstein.net/public_html/wp-content/plugins/e-newsletter/e-newsletter.php</b> on line <b>1592</b>

Any thoughts?