Whenever you're starting out on a new IP address, it's important to slowly and methodologically build a good sending reputation. The IP address is new and unused; it does not yet have a reputation of its own. ISPs are suspicious of mail from new, previously idle IP addresses, as they say that the vast majority of such connections is to attempt to delivery bad stuff, like spam or phishing messages. (Hotmail once told me that more than 80% of time, a new IP address connecting to them is trying to send them something bad.) So they're pretty touchy about that.
It's not that hard to deal with, really. It's just a case of, send only certain volumes at first, and making sure you're putting your best foot forward.
What does that mean?
First, sending only certain volumes. Roughly speaking, send a maximum of 20,000 messages per day to each ISP during the first week. If you have less than 20,000 per ISP, that's okay -- this is a maximum, not a minimum requirement. Double this amount, week over week.
Second, put your best foot forward. What you're doing here is you're "warming up" or "ramping up" this new IP address. You're giving the ISP a chance to gather reputational stats (things like bounce percentage, complaint percentage) on your mail, and the ISP view of these stats has to be positive, or else you'll succeed. Meaning, if you buy a list and try to warm up your IP address, things will go very, very badly for you. You'll never get more than a few messages delivered before getting blocked. Permission is always important, but it's important to highlight that this is an area where the negative feedback due to lack of permission is going to come back to you, very quickly, very severely.
This isn't really new advice, nor is it ExactTarget-specific advice. (Don't take my word for it -- here's another good article on IP address warming.)
Here at ExactTarget, we have an IP address ramp-up guide that goes into more detail on IP address warming best practices. Feel free to reach out to the deliverability team if you'd like to receive a copy.
For more tips and advice to help you hit the inbox, check out our free Email Deliverability Guide.