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:
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:
See also: RFC 1912
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:
See also: RFC 1912
The relaying server gets back the following temporary error message:
See also: RFC 2821
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.