What is IP warming and why is it necessary?

IP warming is the process of gradually increasing the volume of emails sent from a new IP address over a period of time. This helps establish a positive sender reputation with internet service providers (ISPs), preventing your emails from being marked as spam or blocked. ISPs are cautious of new IPs sending large volumes without a history.

How long does IP warming usually take?

The duration varies depending on your target sending volume, list quality, and type of emails (transactional vs. marketing). Generally, it can take anywhere from 2 to 6 weeks, or even longer for very high volumes. It's a gradual process, often starting with low volumes and slowly increasing, typically by 30-50% daily.

What happens if I don't warm up my IP address?

If you don't warm up your IP address, ISPs will likely view your sudden email volume as suspicious. This can lead to your emails being filtered into spam folders, throttled (slowed down), or outright rejected. Your IP may also end up on a blacklist (or blocklist), severely impacting your email deliverability and sender reputation.

Which email contacts should I send to first during IP warming?

You should focus on sending emails to your most engaged and active subscribers first. These recipients are more likely to open, click, and interact positively with your emails, sending strong positive signals to ISPs. Avoid sending to old, unengaged, or purchased lists during the warming period.

What are the best practices and schedules for warming up an IP address for email sending? - Sender reputation - Email deliverability - Knowledge base

Q: How do I monitor the progress of my IP warming?

Monitor key metrics like bounce rates, spam complaint rates, open rates, and click-through rates. Tools such as Google Postmaster Tools and other ISP feedback loops provide valuable insights. If you see an increase in negative metrics, it's a sign to slow down your sending volume and investigate the issue.

Warming up an IP address for email sending is a critical step that often determines the success of your email campaigns. When you start sending emails from a new IP, internet service providers (ISPs) view it with suspicion, as new IPs have no established reputation. Without proper warming, your emails risk being flagged as spam, ending up in junk folders, or even being rejected outright. This process builds trust with ISPs, demonstrating that you are a legitimate sender.

The core idea is to gradually increase your sending volume over a period of time, allowing ISPs to observe your sending behavior and assess your trustworthiness. It's not just about volume, though. The quality of your sends, engagement rates, and adherence to email best practices play an equally significant role. A well-executed IP warming strategy lays the foundation for strong deliverability and a positive sender reputation.

Understanding IP warming fundamentals

IP warming is the methodical practice of building a positive sending reputation for a new or recently unused IP address. ISPs track various metrics, including spam complaints, bounce rates, and engagement, to determine the trustworthiness of an IP. A sudden surge in email volume from an unknown IP can trigger spam filters, leading to poor inbox placement. By starting with small volumes and slowly escalating, you demonstrate consistent, desired sending behavior.

The necessity for IP warming stems from how email systems identify and filter unwanted mail. When a new IP attempts to send a large number of emails, ISPs often mistake this for spamming activity. They have algorithms designed to protect their users from unsolicited mail, and a cold IP sending high volumes sets off immediate red flags. This initial caution is why a measured approach is crucial. You can learn more about warming up your IP addresses in our guide to email IP warm-up approaches.

While both IP and domain reputation are vital, IP warming specifically focuses on the sending IP. Some mail transfer agents (MTAs) and older systems rely heavily on IP reputation. Even as some providers shift more towards domain-based reputation, a strong IP reputation remains fundamental for consistent email delivery. It's the first impression your emails make with recipient servers.

Crafting your IP warming schedule

A standard IP warming schedule involves gradually increasing your sending volume daily or weekly. There isn't a single universal schedule, as it depends on your overall volume, the type of emails you're sending, and the engagement of your audience. However, a common principle is to start with a very low volume to your most engaged subscribers and slowly expand. Aggressive increases, such as doubling your volume every day, can be risky and may lead to deliverability issues.

Many experts suggest a daily volume increase of between 30% and 50% when performance is good. For example, if you send 1,000 emails on day one, you might aim for 1,300-1,500 on day two. This allows ISPs to slowly build trust without being overwhelmed. Consistency is key, so try to send emails daily, even if it's a small volume, throughout the warming period. For more details on scaling your email volume, consider our guidance on IP warming strategy and email volume scaling.

Below is a sample schedule for a 30-day IP warming process, assuming a target daily volume of 1,000,000 emails. This is a guideline, and real-world results may require adjustments based on performance and recipient feedback. This table aims to provide a clear, structured way to approach the ramp-up.

Day	Daily volume
1-3	5,000
4-7	10,000
8-10	25,000
11-14	50,000
15-17	100,000
18-21	250,000
22-25	500,000
26-30	1,000,000

The example above is for a high-volume sender. For lower volumes or transactional emails, the ramp-up might be quicker. Always prioritize sending to your most active and engaged subscribers during the initial phases. You can find more comprehensive guidance, including specific schedules for transactional emails, in Twilio SendGrid's email guide to IP warm up.

Key best practices for successful warm-up

Effective IP warming goes beyond just increasing volume, it also requires adherence to several best practices. First, ensure your email list is clean and verified. Sending to invalid or unengaged addresses will quickly harm your new IP's reputation. Focus on sending only to your most active and engaged subscribers initially, as positive interactions (opens, clicks) signal good sending behavior to ISPs. Avoid sending to purchased lists or old, inactive contacts.

The danger of unengaged lists

Sending to unengaged or outdated lists during IP warming can quickly lead to high bounce rates, spam complaints, and even trigger spam traps. This will land your new IP on a blacklist (or blocklist), making it incredibly difficult to achieve good inbox placement. Always prioritize list hygiene and permission-based sending.

For more information on the impact of blocklists (or blacklists), refer to our guide on what happens when your IP gets blocklisted.

Another crucial aspect is proper email authentication. Before you even begin sending, ensure your SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting & Conformance) records are correctly configured. These technical standards verify your sending identity and are foundational for deliverability. ISPs rely heavily on these to determine if an email is legitimate.

Example DNS records for email authenticationDNS

v=spf1 include:_spf.example.com ~all
v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDyO02v... 
v=DMARC1; p=none; rua=mailto:dmarc_reports@example.com

During the warming process, continually monitor your deliverability metrics, including open rates, click-through rates, bounce rates, and spam complaint rates. Tools like Gmail Postmaster Tools and other ISP feedback loops provide valuable insights. If you notice a spike in bounces or complaints, it's a clear signal to slow down your sending volume. You can find more information on these protocols in our simple guide to DMARC, SPF, and DKIM. Maintaining a positive sender reputation is an ongoing effort, not just a one-time event.

Adjusting your strategy and troubleshooting

While a planned warming schedule is essential, real-world email sending can be unpredictable. If you encounter deliverability issues during your IP warming, the primary response is almost always to pull back and slow down. A sudden increase in deferrals, bounces, or spam complaints indicates that ISPs are not yet trusting your new IP. Reduce your daily volume, focus on highly engaged segments, and give the ISPs more time to build confidence in your sending patterns.

It's also important to analyze the type of issues you are facing. Are you seeing specific error codes, or are emails simply not reaching the inbox? High bounce rates often point to list quality issues, while high spam complaints indicate content problems or sending to unengaged users. Understanding these nuances will help you adjust your strategy. You can find more specific advice in our article about managing deliverability for large sends without sufficient IP warm-up.

Aggressive warm-up

Pros: Potentially faster reach to full sending volume.
Cons: High risk of hitting spam traps, getting blocklisted, or triggering ISP filters. Difficult to recover from poor reputation.

Conservative warm-up

Pros: Builds a solid, reliable sender reputation. Lower risk of deliverability issues and blacklisting (or blocklisting).
Cons: Takes longer to reach full sending capacity. Requires more patience and consistent monitoring.

Remember that IP warming is a delicate balance. It's about communicating your sending intentions to ISPs through consistent, positive sending behavior. While some external resources and service providers offer automated warming solutions, understanding the underlying principles allows for better troubleshooting and adaptation to unforeseen issues.

Views from the trenches

Best practices

Start by sending only to your most engaged subscribers who are highly likely to open and click your emails.

Maintain consistent sending volumes daily throughout the warm-up period, avoiding large, erratic spikes.

Ensure all technical authentication, including SPF, DKIM, and DMARC, is correctly set up before starting.

Segment your list to send to various mailbox providers (ISPs) proportionally to your overall list size.

Continuously monitor your deliverability metrics, such as bounce rates and complaint rates, daily.

Common pitfalls

Doubling your email volume daily can be too aggressive and often leads to deliverability issues and blocklisting.

Sending to unengaged or old lists, which can trigger spam traps and increase complaint rates.

Failing to set up proper email authentication (SPF, DKIM, DMARC) before starting the warm-up process.

Ignoring feedback loops from ISPs, which provide critical information about your sending reputation.

Stopping the warm-up process too early or not maintaining consistent sending after reaching full volume.

Expert tips

Be conservative with your daily volume increases; 30-50% growth is often safer than 100%.

Prioritize engagement: ISPs reward IPs that send emails recipients want to receive.

If you encounter issues, immediately reduce volume and identify the root cause before resuming the ramp-up.

Different mailbox providers might require slightly different warming paces; adapt your strategy as needed.

Don't just warm up for a few weeks; maintain good sending hygiene and consistent volume long-term.

Expert view

Expert from Email Geeks says they typically advise a 30% daily growth in sending volume, pushing to 50% when performance is strong. They noted it's not always a linear scale, and sometimes no growth or a pullback is necessary.

2024-06-10 - Email Geeks

Expert view

Expert from Email Geeks mentioned that some IP warming policies, especially those that involve doubling volume daily, can be too aggressive. They emphasized the need to monitor how things are progressing and know how to react if issues arise.

2024-01-20 - Email Geeks

The path to strong email deliverability

Mastering IP warming is fundamental to achieving high email deliverability. It's a strategic process that involves not just following a schedule, but also understanding the nuances of sender reputation, list hygiene, and technical authentication. By patiently building trust with ISPs through consistent, engaged sending, you set your email program up for long-term success. Remember, a cold IP is a suspicious IP, and only through careful warming can you transform it into a trusted sender.

Ongoing monitoring and adaptation are just as important as the initial ramp-up. The digital landscape for email deliverability is constantly evolving, with ISPs regularly updating their filtering algorithms. Staying vigilant, analyzing your performance data, and being prepared to adjust your sending strategy are continuous responsibilities for any serious email sender.