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Solving mail delivery problems

In case you have received an email error message directing you to this web site this means that your mail has been rejected by one of our servers. The reason for this delivery failure is due to the fact that our mail servers do some tests on the incoming mail. This page is intended to explain the technical background of the protection measures. If you do not understand the following descriptions please contact the mail administrator who is responsible for your mail relay.

  1. Blocked file formats
    We block a variety of executable file formats, Microsoft® Office documents, archive file formats, and similar types of files or data. There will be no exceptions. Please submit these files or the data by alternative means. For text documents, presentations, and exports from spread sheets please use Portable Document Format (PDF) files, preferrably the ISO-certified PDF/A.
  2. Blocking because of blacklisted servers
    If you receive an error message of the type 550 Rejected: listed at then your server is blacklisted. This means it has been added to one of the databases that keep track of misconfigured servers. We do not accept emails from server that have an entry in one of the following databases (this list is not complete and is subject to changes): Selected servers may use additional blacklists. Please take care that your mail server is configured properly and passes all tests that check for ways of abusing your setup. There will be no exceptions for any blacklisted server. Should your server be blacklisted on one of these databases please take the appropriate steps to have your IP(s) removed. You can use suitable tools in order to verifiy if your server is blacklisted.
  3. Blocking because of incorrect or missing DNS entries
    Please take care that your DNS is configured correctly. A mail server must have a valid A record and a valid PTR record. Here is a list of criteria your server will be checked against.
    • The domain of the sender must exist and must resolve.
    • The name of the mail server must resolve to an IP address (A record).
    • The IP address of the mail server must resolve to a name (PTR record).
    • The mail servers that handle mail for a specific domain must exist and must point to valid MX records.
  4. Keep in mind to check your DNS configuration for any errors and make sure that your DNS servers propagate your zone information as fast a possible.
  5. Blocking because of an unfriendly or confused server
    We only accept emails from servers with an acceptable HELO/EHLO according to RFC 2821. Some of our servers additionally check for the presence of SPF records and check for the Sender Rewriting Scheme (SRS) in case of forwarded email.
  6. SMTP Greylisting
    Our servers track SMTP transactions and store the IP addresse and the sender/recipient pair into a database. Every unknown server will be greeted with a temporary error code and will have the mail delivery delayed. This method is called greylisting. All sane software and servers in compliance with standards have no trouble with this.
  7. Invalid format of emails
    Badly written or badly configured software can produce emails with invalid or not parseable content. Some of our servers use content filters that analyse, partly correct or block the message. This is especially true for
    • emails in HTML format
    • emails with special characters in the header (unescaped 8 bit characters)
    • emails classified as spam by trained spam filters
    • emails with known virus or spam signatures
    • emails which contain parts encoded in the Transport Neutral Encapsulation Format (TNEF), it can be deactivated in the mail client. Microsoft suggests that MS-TNEF attachments are only suitable for use within your own company, and recommends that it not be used for Internet mail (see Q241538 and Q138053).
    We accept emails that follow the format definition of RFC 2822. We use DSPAM for analysing emails stored to our local mailboxes. In case you wish your emails to passe our filters unharmed please study the following guidelines with regard to the content of your message(s).
  8. Individually blocked sender addresses and domains
    If our postmasters see error messages connected to specific domains, IP or email addresses in our logs then these origins may be put in our internal blacklist. If you think that these blocks are not correct then please ask our postmasters to remove the origin in question (use the contact address specified in RFC 2821 for telling us).
If you want to test your mail relay you can send a mail to echo at You will then receive a return mail containing the full headers of your posting. The test mail server employs more of the above explained sanity checks. If you received the echo mail your mail system is probably configured properly.