Warming up a new IP in an SMTP cluster is more of an art than precise science. There isn’t a hard andrt fast way to warm up the new IP, although there are a lot of advices on the smtp forums. My favorite warm-up routine is on the slower side. If you are building a new SMTP cluster with a new domain, chances are, you don’t have domain reputation to augment your IP reputation. In this case, you should really start slow with your IP warm-up process.
I usually start with a 2000mail/day plan for each IP. That means, if you have four IPs, you should start sending with 4 x 2000 emails/day from that smtp server. Every other day, this volume can be doubled, as long as you are not being heavily throttled by the recipient SMTP servers. If you are, then you should invest more time in your list management practises. Incorrectly ramping up the mail volume while recipients are complaining can cause your smtp server to be blacklisted!