smtp.web.de fails to send emails

Recently a friend of mine, who uses my mail service, complained about e-mail not being received from users of WEB.DE. The sender received an error mail with following content:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of
its recipients. This is a permanent error. The following address
failed:

  <nix@example.com>

domain name system error:
domain has no mail exchangers