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Technology: Multiple IP Addresses: Why and How Many?


Whether you're new to sending email in bulk, or an experienced mailer switching from an ESP to your own mail server, there are several things to consider regarding the IP addresses associated with the servers you'll be sending mail from.

I'm often asked: How many IP addresses do I need? While there is no one-size-fits-all answer to this question, there are some basic factors to consider in choosing the proper number of IP addresses.

Why do I need multiple IPs?
The first question you need to ask yourself is: WHY do I believe I need new, more, or multiple IP addresses?

In my opinion, there are really only two legitimate reasons for adding new IP addresses: Volume (as it pertains to time), and having multiple e-mail channels or mail streams.
  1. Volume (as it pertains to time): A large volume of messages being sent, in-and-of-itself, does not necessarily justify using multiple IP addresses. I have worked with clients who send over 2 million emails a day from just one IP address and reach the inbox on a consistent basis.  However, if your messages are time sensitive or need to be delivered in a certain timeframe, then it may be necessary to use multiple IP addresses to send the total volume of email needed in the timeframe required. As an example, say you are sending a campaign to 2 million emails at 8am, and the campaign needs to be delivered by noon. If it normally takes 12 hours to deliver 2 million emails from one IP address, then in order to deliver all 2 million emails by noon (4 hours), you would need 2 additional IP addresses (12 divided by 4 = 3 IPs). The amount of time it takes to send out 2 million emails is unique to your situation and will depend on a number of factors, including infrastructure, software, and your current deliverability reputation (as many ISPs will limit and throttle email based on your sending reputation).
  2. Multiple e-mail channels or mail streams: Using different IP addresses segmented based on particular mail streams is another legitimate reason for using multiple IP addresses.  Since each IP address will maintain its own deliverability reputation, segmenting each mail stream by IP address will keep the reputation of each mail stream separate. Some examples of segmenting by mail streams are:
  • Transactional messages
  • Marketing messages
  • Newsletters
  • Addresses derived from "Forward to a Friend" campaigns
  • Reactivation campaigns
  • Less-active or inactive addresses
  • Different brands for each of the above
Why do I not need multiple IPs?
A common reason I hear for wanting to add additional IP addresses is that the sender is not getting the reputation or the inbox deliverability they'd like on the IP addresses they're currently sending from.

This is NOT a good reason to add additional IP addresses. Your reputation will follow you. ISPs are very sophisticated in tracking a sender's mailing patterns and reputation. A lot of this is tied to the IP address, but not all. Many other factors are tracked, such as sending and link URLs, patterns of sending, addresses sent to, and content and engagement, just to name a few.
It is much better to determine the causes of your deliverability reputation woes and fix them, rather than getting new IP addresses to try to run from them. While this process will likely take longer, in the end the results will provide better long-term inbox deliverability.

How many IP addresses are right for me?
If there is a legitimate reason to add or use multiple IP addresses, then here are a few factors in determining how many you need:
  • Budget: In obtaining new IP addresses, each IP address that you acquire comes at a cost.There is an incremental charge for each actual IP address you obtain. This cost will vary. If you are getting an IP address from your ESP, then this could be expensive and result in additional monthly costs. Even if you are using your own MTA and mailing software and obtain your IP addresses directly from your service provider, the setup and installation associated with each IP address (including setting up feedback loops and configuring authentication) have incremental costs.
    However, just because your budget allows for the purchase of additional or multiple IP addresses does not necessarily mean that it's a good idea to use them.
  • Deliverability Reputation: As I mentioned above, if your delivery reputation is poor, switching to new IPs will not solve the issue, and in fact can exacerbate it (more on this below). However, reputation can play a role in determining how many IP addresses to use.If your current deliverability reputation is causing your messages to be throttled, it may make sense to utilize two to three different IP addresses to allow for your messages to be delivered in a timely matter. This needs to be addressed with caution, however. It is a temporary solution for those with a fair to good reputation in the process of improving to a great reputation. It is not a substitute for fixing the issues causing the less-than-great reputation. You can check the ISPs' postmaster pages to see if they publish throttling rates, or you can use past mailing logs to get an understanding of the rate at which your mail is being delivered. Another deliverability reputation factor involved in determining the number of IPs to use is related to segmentation based on mail-stream, as discussed in detail above. If the mail stream or data segment you are about to send to could negatively affect your overall sending reputation, then it may make sense to separate that mail stream from your normal IP address that sends to your most critical list, and instead send it from a different IP address.
  • IP Warm-up Time: When adding or using new IP addresses, you will need to properly warm up the new IPs by sending a smaller amount of email at first, and then slowly increasing the volume over time to avoid appearing as a spammer in the eyes of the ISPs. This process takes time – anywhere from a couple of weeks to a month or more. This time frame should factor into your decision to add new IPs. More information about the IP warm-up process can be found in my previous article titled "Your New IPs Need to Learn to Walk Before They Run".
  • Risks versus rewards: Using too many IPs or switching to new IPs can come with risks.
Here are a few things that ISPs look at that may raise a red flag and get you blocked or placed in the Junk Folder:
  • Snowshoeing – Spammers will try to spread spam output across many IPs and domains in order to dilute reputation metrics in an attempt to evade filters. When an ISP sees that the same message is being sent across multiple IP addresses, it raises a red flag to them that the sender may be a spammer. A basic rule of thumb for good deliverability is to "not look like or act like a spammer". Use only enough IP addresses to meet the requirements of your mailings.
  • Waterfalling – Similar to snowshoeing, waterfalling is when multiple ESPs are used, each ESP having a different IP address range. For each ESP and IP range, the mailer will burn the new IP addresses in the process of cleaning up bounces, complainers and even non-responders. Then they take the new set of good data and move on to another ESP, and they repeat the process like one waterfall flowing into the next, all to avoid high negative delivery metrics on one set of IPs. This is a common practice used by gray mailers and spammers.If, as a legitimate email marketer, you switch to or use different sets of IP addresses, the ISP may see a similar pattern as those used in waterfalling. Again, this may mean you "look like, or act like a spammer."
  • IP Hopping – Jumping from one IP to another, or rotating through a set of IP addresses, can also be seen as suspicious behavior by ISPs. After a few days of this, the IPs get "soiled" and will likely get blocked by the ISP. New IP addresses need to be warmed up properly. It takes time to build up a good reputation. Keep in mind that spammers are notorious for changing IP addresses often.
There are good and not so good reasons for using multiple IP addresses. Determining if the use of multiple IPs is right for you greatly depends on your particular needs and current situation