\ ˈī \ \ ˈpē \\ ˈwȯrm \
IP warming is the process of you letting your ISP know, through various means, that you’re a trustworthy sender, and not a nasty spammer.
If your brand hasn’t sent a significant volume of email in the past, ISPs have no way of knowing who you are. There’s no sending behavior, pattern, or history to see. So, over a period of 4-6 weeks, and sometimes longer thoughtful marketers warm the IP addresses associated with their email sends in order to build their reputation with the ISPs that make delivery possible.
They start by slowly scaling up the volume of emails they send, keep the frequency of their mailings in check, and make sure the vast majority of email addresses on their mailing lists are clean, valid, and confirmed via double opt-in or active subscriptions, among other steps. If your sender reputation is low, you could find legitimate, wanted messages getting filtered into recipients’ spam folders or blocked entirely. And if you don’t properly warm up your IP address, and suddenly start sending loads of messages in a short time frame, it could take weeks (or even months) to recover.
“When we tried to send our holiday campaign without going through the IP warming process, a big chunk of the messages we sent ended up our customers’ spam folders.”