That’s an SMTP error message; it originates from your mail server. At this point, Zammad is probably trying to send a test mail (not sure where to - either to emailtrytest@znuny.com to test outgoing SMTP in general, or to the helpdesk2@… address you’ve configured under “E-Mail-Adresse” to ensure that your Zammad is reachable), and the SMTP server rejects it.