Some of the top ISPs are putting new IP addresses with no sending history on a short leash until they can gather enough reputational data to determine how to deliver the email. In order to establish a reputation on a new IP address, you will have to do the following:
1. Determine your typical volume. Start small by sending batches of only a few thousand per day. After a week, ramp it up to a few thousand per hour until you are able to send your typical volume of email messages. You can determine these thresholds by using tools like Return Path's Mailbox Monitor and Microsoft's SNDS.
2. Verify that your MTA is also configured for each ISP connection and throughput settings. Each ISP has their own specific rules on how many connections a sender can have and how many messages you can deliver during a specified time period. If you fail to adhere to them, you risk your mail not being delivered or accepted.
3. Make sure your new IP is authenticated, whitelisted and set up on all available feedback loops. Continually monitor your mail server's log files to see if ISPs are returning any policy related bounce messages. Also check for complaint rates, spam trap rates and any reputation score changes.
Whether or not you decide to add a new IP address, you may still meet with some resistance. When sending your messages, you may want to consider deploying during "off peak" hours. With the increase in volume at all ISPs this year, more senders will experience the dreaded "too busy, try again later" message. By sending between the hours of 12:00 AM to 5:00 AM, you can avoid some of the gridlock and experience a little less competition for the prime spot in the inbox.
Source: Return Path.