If an IP address does not have a history of sending [large amounts of] email it is considered "cold". If that IP address then suddenly begins sending huge numbers of emails, many providers will assume the worst (that the email is being used for SPAM) and either block the IP or mark messages coming from it as SPAM. "Warming up" the IP means sending a smaller number of emails (per time period) and slowly increasing the volume so that Email providers can properly "score" the IP as legit and okay in their systems.
Update: For some reason I couldn't reply to your response kstrauser so here is my reply: That works if and when their is trust. First you have trust the buyer isn't going to abuse the system and wreck the IP [reputation] and you have to trust the buyer isn't giving you stolen info. Obviously things one can work through but again, it involves trust. Also the provider has to have warmed up IPs to give out. Which, having warmed up IPs would actually be a great valued add upsell for those that need them!
Update 2: Thank you FfejL and symfoniq for explaining it better.
That's when you physically call your sales rep and explain why your using their services and ask them to whitelist you. I say this from experience. If you called my employer (and any of our competitors, I'm certain) and said "I need to host a mail relay that's ready to go from no traffic to tens of thousands of emails per second, here's my company website, and here's our credit card", we'd be happy to make it work for you.
I'm not saying this to advertise but to offer a suggestion: call someone and ask for help. This isn't an unusual need at all and any reputable provider will be quick to help.
Short version: you don't have to "warm up an IP" if you do a little advance homework.
If Google (or any other ISP) suddenly sees an big spike in email from @example.com on an IP that @example.com hasn't used previously, Google is much more likely to mark those emails as Spam.
So senders need to 'warm up' an IP by sending a small amount of email first, usually for a few days at least.
So if you're a SendGrid customer expecting to send 2,000,000 emails today, you can't just switch to a new ESP and send those same emails. Spam rates will go through the roof.
jrs235 is referring to IP reputation as measured on the receiving end of the email. In other words, Yahoo or Hotmail might reject your emails if they see unusual activity from an IP address. Until an IP is "established" as having sent a lot of email with low bounce and spam rates, getting bulk email delivered from that IP address will be a frustrating experience.
Mail me at kirk@strauser.com . I'm not trying to be evasive but I like to keep work and personal separate, especially since this story has become more political than I was expecting. I just came in to read about a service outage.
