Email Policy in the context of Antivirus/AntiSPAM service

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.

    Deutschland GmbH