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Email servers with generic hostnames
For many email servers you will find the required reverse lookup in the DNS. Unfortunately many of these hostnames are generic hostnames. That means they contain the IP address of the host (like xxx-xxx-xxx-xxx.reverse.provider.tld) or they are numbered serially (like host-xxx.provider.tld).
Such generic host names are a usually an indicator for dynamic assigned IP addresses. Due to this many email servers reject the mails and expect the delivery of the email by an email server with a static IP address.
Also networks with static IP addresses but generic hostnames can be found. This are often networks with quickly changeing customers (like virtual host center). Due to this email servers with a static IP address but a generic hostname are equated with dialup networks. One reason is that it is often not possible to make a difference. And even if it is possible many email server expect a individual hostname as indicator of a real email server business.
We suggest to have your system administrator or e-mail-provider update your entry in the DNS-configuration. You might use mail.yourcomanyname.tld for example. Please do not forget to add the hostname itself to the DNS and set it as HELO of your mailserver. You avoid problems with the delivery of your e-mails.