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.
- 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.
- Blocking because of blacklisted servers
If you receive an error message of the type 550 Rejected: ip.ad.re.se listed at server.domain.la
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
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.
- Blocking because of incorrect or missing DNS entries
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.
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.
- 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.
- 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
sane software and servers in compliance with standards have no trouble with this.
- 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
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).
- 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
- 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 luchs.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.