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Rejected E-Mails

Deutsche Version

You have probably reached this page because of an email that was blocked, resulting in a non-delivery notification (NDN, bounce).

Please notice:

  1. NetCologne doesn't block mails arbitrary, but is forced to block to protect our infrastructure and to offer best and performant email services to the customers.
  2. A removal from the blacklist can only be achieved by the ISP resonsible for the blocked IP address! NetCologne cannot act on removal requests by third parties, because only the ISP itself is able to detect and fix the problem.

This are the main reasons for emails being blocked, and what to do to avoid them.

  • Greylisting:
    Some systems are using greylisting, which may lead to a little email delivery delay during the initial contact. After this, all further mails should be whitelisted. If you see an error message saying " Please try again in a few seconds",ask the postmaster of your mail system to configure it in a RFC- compliant way.
  • Use of a dynamically assigned IP address:
    Dynamically assigned IP addresses are blocked. This can be seen in the error message "source: Dyn/Dialup". In this case, use the mail server provided exactly for this reason by your ISP.
  • Use of an IP address without reverse DNS:
    The domain doesn't accept IP addresses without reverse DNS for delivery. The error message is "cannot find your reverse hostname". Please ask your ISP to set up a reverse DNS entry for the respective IP.
  • Use of RBL:
    Error-Code: 552 5.7.1 unknown[ip-adresse] was blocked due to being a dynamic IP. See for more info. ) or 554 5.7.1 Service unavailable; Client host [ip-adresse] blocked using xbl.dnsbl; ?ip=ip-adresse RBL are "Realtime Blackhole Lists" which are used to block specific IP addresses quickly and effectively. Actually, NetCologne uses the XBL, the SBL and the PBL ("Policy Block List") from Spamhaus. In this list, ISPs can maintain themselves the networks which are not used to send mail, so that other ISPs can block them safely. NetCologne also contributed the dynamically assigned IP ranges to this list. Please ask your ISP whether the IP is perhaps listed wrongly. If not, it should be able to provide you with a mail server which is not blocked.
  • No RFC-compliant HELO:
    A RFC-compliant HELO is mandatory. Otherwise, the mail will be rejected with the message "504 Helo command rejected: need fully-qualified hostname:". Please contact the postmaster of the mail system you are using, so that he may configure the system in a RFC-compliant way. (See RFC 2821.)
  • Use of spam sending servers:
    Error-Code: 554 5.7.1 Service unavailable; Client host [ip-address] blocked using ncblack.dnsbl; IP . You did send SPAM or VIRUSES to us in the past. Please see In case of massive spam, mostly combined with the responsible ISP not providing any abuse contact or being just ignorant regarding long lasting complaints, such servers may be blocked via a manually maintained blacklist ("source: net abuse" etc.). Please ask your ISP to contact to fix the problem. Please notice as an ISP, that blocking abuse complaints due to "spam" is completely free of sense, and that for good reason, you never should filter your abuse address! In case of missing, wrong or inactive abuse-addresses, where we cannot report incidents which might affect our network we keep the right to block the offending ip-addresses. (Error-Code: 554 5.7.1 Service unavailable; Client host [ip-adresse] blocked using ncblack.dnsbl; You have no (working) abuse contact.
    Please see
    (See also: RFC 2142.)
  • Honeypot-Attackers
    Error-Code: 554 5.7.1 Service unavailable; Client host [ip-address] blocked using rbl.frickl; successful attacker of a NetCologne honeypot; You are trying to send mail from a host which has successfully attacked a NetCologne Honeypot or which has sent spammails to a NetCologne Spamtrap email address. This block entry happens automagically; if your host does not exceed a certain threshold due to further attacks, the block will be released after 24h.