Static IP is allocated to an ISP client under assurance that such IP will never be changed. This warranty is given under a contract between the parties. In contrast, dynamic IP is allocated to the client without any guarantee that it will not be changed by the ISP.
Some people believe that the dynamic IP is one that is constantly changed by the ISP. But this is not the most correct definition, as there may be cases where the ISP maintains the same IP in a residential link, for technical reasons. The exact definition of dynamic IP is by no contractual guarantee that such IP will never be exchanged for a given link.
How to identify a dynamic IP
All dynamic IPs necessarily have a generic rDNS or no rDNS. When exists, the rDNS domain must be in the name of the same ISP that provides IP. If the ISP provides residential access to the Internet, then the IP is considered dynamic.
Some ISPs configure rDNS to make it clear that such IP is dynamic. They can use words in the composition of rDNS as: dyn, dynamic, dialup, ftth (fiber to the home), mobile, etc. Some examples of dynamic rDNS:
It can also occur ISP leave described the particular IP block is being used for residential purposes, through the WHOIS system:
ubuntu:~$ whois 126.96.36.199
ubuntu:~$ whois 188.8.131.52
descr: JSC "ER-Telecom Holding" Kursk branch
descr: Kursk, Russia
descr: PPPoE individual customers
Use of dynamic IP on email server
The use of dynamic IPs is prohibited, by international standards, in e-mail servers. In addition, MAAWG recommends in its document that port 25 be blocked for dynamic IPs by the ISPs themselves. For some reasons, some ISPs do not follow such a recommendation, leaving port 25 open and free for malicious programs to use dynamic IP for SPAM or fraud. Several DNSBLs offer dynamic IP listing services because of this lack of ISP control.
It may occur that your mail server is using a dynamic IP with port 25 opened. In this case, the simple fact that port 25 is open does not qualify such IP to host e-mail service. If your e-mail server is using a dynamic IP, contact your ISP for static IP hiring and FCrDNS setting. This will prevent other e-mail servers from refusing your messages by dynamic IP checking.