Error Messages Caused by Undeliverable Email

You have sent an email and received a notification that your email has not been delivered to the recipient. You can recognize such emails by the sender Mail Delivery System and the subject Mail delivery failed. These messages are often referred to as Bounce, Non Delivery Notification (NDN) or Non-Delivery Report/Receipt (NDR).


  • 550 Unknown User: Address incorrect. The email recipient is not known. This is not an error, but the destination address used is incorrect. Check the address for typing errors or contact the owner.
  • 550 Requested action not taken: mailbox unavailable: The email address does not exist on this server. Check the address for typing errors.
  • 550 Quota Exceeded: The recipient's mailbox is full. Contact the recipient and inform them to make room in their mailbox.
  • 535 Authentication failure: SMTP AUTH failed with the remote server: Our email servers are not open relays. If you would like to use our email servers to send emails, it is always mandatory to specify a user and password. How and where this is set in which program or web service cannot be answered by us. However, there are programs or services in which authentication (password specification) is not built in.
  • 554 Service unavailable; Client host [...] blocked using - IP Black Listed: In this case there is a blacklisting. All emails sent by one of our email servers are rejected by the receiving server. Black listings often occur only temporarily. In the meantime, resend your email. If it takes longer, please contact our customer service.
  • Your message couldn't be delievered to all recipients The following recipients are not reachable: '' 550 must be authenticated: Your message was not delivered to all recipients. The recipient'' cannot be reached:

    The error code 550 is very general, so there is no direct solution. The following features can lead to error 550:
    - Own mail server with dyndns and connection to the Internet, this is rejected by almost all providers because of SPAMMING;
    - Authentication is required for sending emails;
    - E-mail client users may not activate SPA authentication (login using secure password authentication required);
    - Relaying is not allowed.
  • domain has no valid mail exchangers: This problem can be solved by adjusting the mail server DNS settings. According to the current RFC standard 2181 10.3, mail server DNS entries (the so-called MX entries) must resolve to a unique address and may not forward to an alias address, a CNAME.