ERROR MESSAGE:
Could not deliver message to the following recipient(s):
Failed Recipient: carolin.miltner@germany.travel
Reason: Remote host said: 451 4.7.1 Your IP address does not resolve to an existing hostname - no proper
reverse mapping exists - see http://emailpolicy.interoute-deutschland.de/
-- The header and top 20 lines of the message follows -Received: from Chris (rrcs-76-79-187-130.west.biz.rr.com [76.79.187.130]) by mail.si-sv2090.com with SMTP;
Thu, 15 Jul 2010 14:47:00 -0500
From: "Chris Weike \(Nonstop Travel\)" <chris@nonstoptravel.net>
To: "'Carolin Miltner'" <Carolin.Miltner@germany.travel>
References: <4C3B4BE4.0508.001C.0@germany.travel>
Subject: RE: Workshop Overseas 2010
Date: Thu, 15 Jul 2010 12:46:58 -0700
Message-ID:
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AAAA==@nonstoptravel.net>
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Hi Caroline,
=20
Thanks for the info.=20
=20
ANSWER FROM THE IT PEOPLE AT THE GERMAN TOURSIM BOARD:
Reference about SMTP errors
If an email message is being rejected by our systems we always send a meaningful error
information while the communication is being canceled. The following terms describe
SMTP error handling and error messages in respond to common mistakes within the scope
of operating an email server:
 No reverse mapping (PTR Record) exists
IP addresses of all mail servers have to resolve to a hostname.
Example:
nslookup 194.231.55.234
234.55.231.194.in-addr.arpa name = relay.vianetworks.de.
This IP address successfully resolves to the fully qualified domain name
relay.vianetworks.de. Furthermore the name relay.vianetworks.de itself does resolve to
right address (forward mapping). In this case the reverse mapping perfectly matches to the
forward mapping. If a reverse mapping exists but in turn that hostname resolves to another
IP address the reverse mapping is inaccurate and thus broken.
Example:
nslookup relay.vianetworks.de
Address: 194.231.55.234
The sender gets back the following error message:
Your IP address does not resolve to a hostname - no proper reverse mapping exists
See also: RFC 1912
 Inappropriate reverse mapping / dynamic IP addresses
The delivering server has a reverse mapping which is not plausible for a mail server or it uses an address located in
a dial-in pool.
Example:
nslookup 217.255.247.59
59.247.255.217.in-addr.arpa. 86400 IN PTR pD9FFF73B.dip.t-dialin.net.
The result shows a host located in a typical dial-in network range used by ADSL or ISDN nodes. Emails sent by this
host are rejected because on the one hand it is a dynamic IP address and on the other hand the reverse mapping is
not appropriate to a mail server.
To fix this problem the operator has two options. He may establish a meaningful reverse mapping for the servers IP
or in the case of a dynamic address he should use a well-engineered email system to send mails, perhaps the one of
his internet provider.
The sender gets back the following error message:
You are a dynamic host - please use your providers mailserver to send emails
See also: RFC 1912
 Greylisting
We use several techniques to fight spam. One quite successful method is called greylisting. It uses a mechanism
which temporary rejects the very first attempt to send an email for a few minutes. No error message will be sent to
the email client. After these few minutes the email is accepted for delivery while the sending server repeats the
delivery. Any further emails which show the same characteristics are aceepted immediately from now on for at least
ten days.
This method proved to be efficient especially in times of high volume spam rates. The only disadvantage is the
slightly higher delay of about five to twenty minutes for the first delivery which is more than balanced by
advantages. Functional servers which agree with existing SMTP standards are known to produce very small delays.
The relaying server gets back the following temporary error message:
Greylisting in action, please come back in a few minutes
See also: RFC 2821
 Blacklists
Several blacklists, both publicly known and internal lists, with lots of spam sending addresses proved to be a good
measure against spam. While blacklist generated false positives are rare they provide a good basis to decrease
ambient noise in terms of unsolicited bulk e-mail. If you think your server is blacklisted without a reason please
contact the provider of the blacklist mentioned by the error message. If the error message just indicates the
following feel free to contact us:
We don't accept mail from your IP
If, in the event of failures, the sender of an email message does not receive one of these or
other more common error messages, the sending server system is likely to have even more
problems apart from the drawbacks mentioned above. In this case the local administrator
should verify the systems error handling too.