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E-Mail-Server without reverse-lookup
Unfortunately, for some email server there is no possibility to resolve a hostname by their IP-address. Technically spoken, these server have no valid reverse-lookup (PTR record in in.ad dr-arpa). Especially when using an own email server on a low priced root-server or a PC with dedicated-line (SDSL) this is often forgotten.
It is also important, that the result of the reverse-lookup matches the IP-address, which means resolving the hostname yields in the originally enquired IP-address.
This kind of insufficient configuration is an attribute of many machines sending spam-mail. Therefor more and more email server (i.e. big ISPs such as AOL, click here for more details) decline e-mails sent from a server without reverse-lookup.
Completing we would like to say, that it is strongly recommended in RFC 1912, paragraph 2.1, to have a PTR record for every single IP-address.
We suggest to have your system administrator or e-mail-provider conduct such an entry in the DNS-configuration. You avoid problems with the delivery of your e-mails. Please be aware not to use generic hostnames for your mailservers.