IP permanence is the process of consistently sending email from an IP address over a period of time in order to establish and maintain a sending reputation.
Mailbox providers look at the length and quality of sending history as one of the factors for filtering email because many spammers frequently change IP addresses in an attempt to evade spam filters. An IP address with a long sending history and good sending metrics helps to establish trust (or sending reputation) with a mailbox provider that the email being sent from the IP address is legitimate and not spam.
