IP warming

  • Updated

New sending IPs have no sending history and are considered a cold IP. At this point, they need to acquire a good reputation with ISPs.

Hotmail, Yahoo!, AOL, Gmail need to see that you are sending important, consent-based messages to permit your mailings into your recipient's inbox.

IP warming is the process used to build a good reputation with ISPs. ISPs monitor your IP to track your sending behavior and are likely to accept your messages based on a good sending reputation.

The IP warming process includes sending consistent, low volume email campaigns, gradually increasing this volume over time until you reach your full volume capacity. ISPs then gain confidence in you as a sender.

It can take 4-6 weeks to complete the IP warming phase. This depends on your volume amount and how well your recipients engage with your messaging.

During the warming process, ISPs set receiving limits on your IP until you meet their standards of good reputation.

Image:  IP Warm Up and Reputation

Getting started with IP warming

  • Allowlist the Optimizely sending IP with your internal corporate network to ensure emails are not blocked by your internal spam filter.
  • Ensure your IT team updates your DNS authentication records.
  • Clean your data using segment and email validation tools. Warm your most active users only during IP warming phase. Active emails addresses consist of those who have recently opened, clicked an email in the last 9 months or opted-in to your mailings using a double opt-in database. Focus primarily on recipients who are less likely to report spam complaints. Never send to a bought or an old, unverified email list.
  • If you don’t have engagement data to identify active email address, focus on recent signup data.
  • Create email content that encourages recipient engagement and that is specific to the recipient's interests. See also: Email content best practices.
  • Send a welcome email to your recipients. This reminds your recipients that they have opted-in and generates engagement for your first send out.
  • As a new sender, it is normal to see some blocking and bulking in the first instance of sending. ISPs are trying to determine your legitimacy. As you continue to send to good, active email addresses and increase user engagement, bulking should end.
  • Review your engagement data (opens, clicks). A decrease in engagement can lead to your messages being bulked by the ISP. Remove recipients from your marketing campaigns who are consistently not engaging.
  • Frequency is important. ISPs like to see consistency in your sending and user engagement metrics.
  • Reputation systems store reputation history for 30 days. Ensure that you do not go without 30 days of sending from an IP. Doing so may require restarting the warming process.

IP warming schedule

Following an IP warming schedule assists in gradually establishing a reputation with ISPs as a legitimate email sender. Start with small email volumes, and gradually increase volume each day according to the two-week set schedule. The following schedule suggests a maximum of 40 percent to 100 percentage increase in volume per day. This however greatly depends on factors including list hygiene, how engaged your recipients are, and spam complaints among other things.

Image: Example IP warming schedule

Day Daily sending volume
1 500
2 1000
3 2500
4 5000


6 20000
7 40000
8 60000
9 100000
10 150000
11 200000
12 250000
13 300000
14 400000
  Continue to double your volume until you have reached your target daily volume.

Tips during the IP warming process

  • Ensure that you are sending consistently and keep to that frequency.
  • ISPs like AOL want to see spam complaints less than 0.2%. Review email engagement on each send and remove recipients who are not opening.
  • Once your IP exceeds the threshold set by the ISPs, the ISP may start to soft bounce the emails. If you experience a higher than normal soft bounce rate, stop sending and re-start the send later in the day.
  • Start the day with recipients who are very likely to open your emails.

What can cause delivery issues during IP warming

  • High spam complaints. This occurs when a recipient clicks "this is spam" button which is shown within the email client.
  • Invalid email addresses (hard bounce).
  • IP address and domain blocklistings on RBLs (public blocklists).
  • Spam traps located within your data list.
  • Unexpected volume increases.

What ISPs like to see

  • Consistent opens and clicks
  • Consistent email frequency
  • Email best practices are in place, as detailed in Deliverability best practices.
  • Emails are authenticated (SPF, DKIM, DMARC)

Warming a sending domain

A sending domain requires a good sending reputation for messages to enter your recipients’ inboxes. Domain warming is similar to IP warming, as it represents mailing practices and volume. The same warming approach mentioned above can be applied to warming up sender domains. The difference however is that domain warming focuses on recipient engagement, which is the key factor to focus on.