IP Warming — Email Marketing to Beat ISP Filters

Christopher London
5 min readApr 10, 2020

If your email marketing campaigns aren’t working like you imagined, you might be overlooking IP warming as part of the process.

Marketing is all about building a reputation and maintaining it by following the best practices. You can be persuasive or have the foot-in-door pitch, but only after you have built enough trust and recognition amongst your customers.

In the case of email marketing, the reputation is built when your emails land in the inbox of your subscribers (deliverability), have the consent of your subscriber(permission-based) and address the needs of your subscribers (personalized).

When it comes to sending marketing emails to interested prospects, most marketers only care about deliverability. So, many commit the mistake of sending multiple email campaigns, as soon as they have collected email addresses from prospects, set up an email campaign, and have appropriate landing pages in place.

What they tend to overlook is the fact that ISP filters are constantly monitoring email traffic to identify suspicious activity. If an IP address, with no history, suddenly starts sending a significant number of emails, it comes under their radar.

The resulting action can be anywhere in the wide spectrum from restricted email flow to IP ban.

Hence the number of emails getting actually delivered from a specific IP, instead of ever-increasing, starts to drop.

This is why it is ever important to conduct an IP warming exercise every time you are sending emails through a new IP address.

IP warming is a methodical process of gradually increasing the email volume sent from a new IP address over a period of several weeks to reinforce a positive sending reputation with the ISP. This is somewhat similar to interacting with new people. You need them to gradually warm up to you to build trust, else they might be overwhelmed by being exposed to your true self.

Assuming that you are planning to send 90,000 emails/month (i.e. 3000 emails/day), it would be a great idea to start with sending ~7,000 emails (i.e. 1000 emails/day) for a week from the new IP and gradually increasing it in the following weeks.

Hence a good IP warming practice looks like:

In case you are wondering when should you consider IP warming, it is possible under two scenarios:

  • Shifting from Dynamic IP to a Static IP: Any ESP (Email Sending Provider) chooses from a wide range of IP addresses to send your email campaigns to ensure maximum deliverability. So, the ISP filters are already warmed up for that range of IP addresses. When you shift to a static IP address, you need to manually warm it up yourself.
  • Migrating to new ESP: When you migrate to new ESP, you shall be using a different range of IP addresses to send your emails. If the number of emails sent exceeds the existing volume for the ESP, ISP becomes suspicious.

Now that you are “warmed up” (pun intended) to the concept of IP warming, you need preparation before you begin. Jumping ahead to sending emails for IP warming, with no safety measures implemented, is similar to jumping off a plane without a parachute. You will hit hard ground.

Some of the prerequisites before you begin IP warming are:

  1. Authenticate your new IP with the correct DomainKeys Identified Mail (DKIM)
  2. Update the IP address in your Sender Policy Framework (SPF)
  3. Ensure the new IP is not set up as an open relay
  4. Map the hostname to your new IP address using CNAME and A Record for an easy reverse DNS lookup
  5. For Outlook.com, you need to also register your IP address in Microsoft Smart Network Data Services (SNDS)

Once you have set up the necessary authentications, you can move ahead to the actual warming up process. The goal would be to send emails that would garner maximum engagements without any complaints. The steps list out how to begin IP warming.

  1. Start with identifying your best performing email. An email that already has a better engagement rate compared to others, has a better chance of increasing engagement. Welcome emails are a good place to start with. Not only are they expected when someone subscribes, but the call to action in a welcome email is also more likely to be clicked.
  2. Settle for the right sending volume and frequency. The goal is to send the same number of emails to a fixed number of subscribers for a few days in a row and then gradually increase your volume.
  3. Monitor the metrics post send. Keep a close eye on the open rates, click rates, bounces, and spam complaints. Aim for 20% open rates at least when warming an IP. Weed out any bounces, invalid entries, and unsubscribes after every email send and replace with new entries to maintain sending volume.
  4. Once the ISP is accustomed to your intended sending volume, you have successfully warmed the new IP

The more consistency you maintain in terms of sending volume, frequency, reduced complaint & bounce levels, the less time it will take to establish a positive sending reputation.

As we wrap up, remember the following key takeaways:

  • Sender Reputation is restricted to the domain you use. It doesn’t migrate when you migrate to a new domain
  • IP warming is mandatory if you are shifting to a static IP or migrating ESPs.
  • Send a consistent volume of emails for a few days and gradually ramp it up till you reach your desired sending volume.
  • Monitor your metrics for open rates, click rates, bounces and unsubscribes after every email sent
  • Be sure to authenticate your new IP address by signing your emails with DKIM and registering it on SPF



Christopher London

Chris London, Owner/Marketing Director for Pixel Productions Inc, is an expert in brand building, Ecommerce Development, and online marketing.