What are the best practices for email IP warm-up when switching to a new ESP?
Michael Ko
Co-founder & CEO, Suped
Published 19 May 2025
Updated 18 Aug 2025
7 min read
Moving to a new Email Service Provider (ESP) is a significant step for any business that relies on email communication. It promises new features, better performance, and potentially cost savings, but it also comes with a critical challenge: maintaining your sender reputation. A key part of this transition is IP warm-up, a process essential for establishing trust with mailbox providers (ISPs like Gmail, Outlook, and Yahoo). Without a proper warm-up, your emails risk being flagged as spam, ending up in the junk folder, or being rejected entirely. I've seen businesses inadvertently damage their deliverability by rushing this crucial phase.
IP warm-up, in essence, is the gradual increase of email volume sent from a new IP address. Mailbox providers monitor the volume and engagement associated with an IP. A sudden spike in sending from a previously unknown IP can trigger spam filters, as it resembles the behavior of spammers. It's about slowly building a positive sending history.
This process applies whether you're transitioning to a dedicated IP address with your new ESP or utilizing their shared IP infrastructure. While shared IPs already have some reputation, your domain's sending patterns still need to be established in conjunction with that IP. Understanding and implementing a sound warm-up strategy is paramount to a smooth migration and sustained email deliverability.
The importance of IP warm-up
When you switch ESPs, you're essentially starting fresh with a new sending infrastructure, which often includes a new IP address. ISPs don't recognize this new IP and need to learn about your sending habits. This is where sender reputation comes into play, and why a thoughtful IP warm-up is non-negotiable.
Sender reputation is your credibility score with ISPs, influencing whether your emails land in the inbox or the spam folder. Factors like sending volume, bounce rates, complaint rates, and engagement (opens, clicks) all contribute to this score. A sudden, large volume of emails from a new IP can signal suspicious activity, leading to immediate blocklisting (also known as blacklisting) or delivery issues.
A well-executed IP warm-up allows you to slowly introduce your new IP to ISPs, demonstrating consistent, positive sending behavior. This incremental approach helps build a strong sender reputation from the ground up, ensuring your legitimate emails reach their intended recipients. It’s a foundational step to secure your email deliverability during an ESP migration.
IP warm-up timeline and volume
The duration of an IP warm-up varies based on your total sending volume and the engagement level of your list. It typically ranges from a few weeks to several months. Starting with a very small volume and gradually increasing it is key. For example, some recommend starting with as few as 500 emails on the first day and incrementally increasing from there, often by 30-50% daily.
Start small: Begin with a minimal volume, typically a few hundred emails, to highly engaged subscribers.
Gradual increase: Increase sending volume by a consistent percentage each day, such as 30-40%. Avoid drastic spikes.
Monitor performance: Closely watch deliverability metrics, including open rates, click-through rates, bounces, and complaints. Adjust your sending pace if you see issues.
Key considerations for a successful warm-up
A successful IP warm-up hinges on several key factors beyond just volume. First and foremost, you need a highly engaged and clean email list. Sending to inactive or invalid addresses will immediately harm your new IP's reputation, potentially landing you on a blocklist. Before migrating, take the time to clean your list, removing bounces, unengaged subscribers, and any potential spam traps. Targeting your most active subscribers during the initial warm-up phase ensures high engagement rates, which send positive signals to ISPs.
Another critical aspect is email authentication. Before you even begin sending, ensure your domain's DNS records are properly configured for SPF, DKIM, and DMARC. These protocols verify that your emails are legitimate and prevent spoofing, playing a significant role in establishing trust. Your new ESP should guide you through this setup, but it's essential to confirm its accuracy.
Whether you choose a dedicated IP or a shared IP from your new ESP also influences your warm-up strategy. With a dedicated IP, your reputation is solely your own, requiring a full warm-up from scratch. Shared IPs, while potentially faster to warm, mean your reputation is tied to other senders using the same IP, which can be a double-edged sword.
Dedicated IP
A dedicated IP offers complete control over your sender reputation. It's like having your own dedicated highway lane for email traffic.
Full control: Your sending practices directly impact your reputation.
Longer warm-up: Requires a meticulous, gradual warm-up process from scratch.
Higher volume: Ideal for senders with consistent high email volumes.
Shared IP
Shared IPs mean you share the sending reputation with other ESP clients. It's like being on a public bus- you're affected by other passengers.
Faster warm-up: Some initial reputation exists, speeding up the process.
Shared risk: Your deliverability can be impacted by the sending habits of others on the same IP. Sometimes you still need an IP warm-up even with shared IPs.
Lower volume: Suitable for businesses with lower or intermittent email volumes.
Practical steps for IP warm-up
The core of any IP warm-up is the gradual increase in sending volume. Begin by sending to your most engaged subscribers, those who consistently open and click your emails. This helps generate positive engagement metrics early on. As your reputation strengthens, you can slowly expand your audience to include less engaged segments.
Maintaining consistent sending practices throughout the warm-up period is also crucial. Avoid drastic changes in email content, frequency, or volume that could confuse ISPs. It’s important to send compelling, relevant content that encourages engagement and minimizes complaints and unsubscribes.
Constant monitoring of your sending performance is perhaps the most important practical step. Utilize your ESP's analytics, Google Postmaster Tools, and blocklist (blacklist) checking tools. Pay close attention to bounce rates, spam complaint rates, and inbox placement. If you see negative trends, it’s a signal to slow down your sending volume and address the underlying issues.
Here's a generalized example of an IP warm-up schedule. Keep in mind that this is a template, and your actual pace will depend on your specific audience and performance metrics. You might need to adjust based on real-time feedback from ISPs.
Day Range
Daily Volume
Target Audience
Days 1-3
500 - 5,000
Most engaged subscribers (opened/clicked in last 30-60 days)
Days 4-7
5,001 - 25,000
Recently engaged subscribers (opened/clicked in last 90 days)
Days 8-14
25,001 - 100,000
All engaged subscribers (opened/clicked in last 180 days)
Days 15+
100,000+ (Gradually scale to full volume)
Full list, including less active segments. Continue monitoring closely.
Avoiding common pitfalls
One of the most dangerous pitfalls during an ESP migration is attempting to send excessively high volumes from a new IP on the first day. I've heard stories, and directly seen instances, where an ESP suggested starting with hundreds of thousands of emails right out of the gate. This is a recipe for disaster. ISPs will view such a sudden surge with extreme suspicion, likely leading to immediate reputation damage and severe blockages. Always start small and build incrementally.
Another common mistake, and a particularly egregious one I've encountered, is being advised to use your new ESP's domain for sending, rather than your own. This is fundamentally flawed. Your domain is your brand identity and carries your established sender reputation. Relying on an ESP's domain can confuse mailbox providers, decouple your domain's positive reputation from your sending, and make it incredibly difficult to manage your brand's standing in the long run. Always ensure you are sending emails using your own authenticated domains or subdomains.
Neglecting to continue monitoring your sender reputation and deliverability metrics after the initial warm-up is also a pitfall. Deliverability is an ongoing effort. Regular checks of your inbox placement, complaint rates, and IP blocklist status (or blacklist status) should become standard practice. Consistency in sending volume and maintaining high engagement are continuous requirements to preserve a healthy sending reputation with your new ESP.
Bad advice from your ESP?
If your ESP suggests starting with hundreds of thousands of emails on a brand new IP or tells you to use their domain for sending, be highly skeptical. These practices are generally contrary to established email deliverability best practices and can severely impact your sender reputation. It's crucial to trust your instincts and question advice that seems off-base. Prioritize a slow, methodical warm-up that builds trust gradually.
Views from the trenches
Best practices
Start with very low volumes, gradually increasing daily by 30-40% to prevent reputation issues.
Segment your most engaged subscribers for the initial warm-up period to ensure high engagement.
Always use your own domain, ensuring proper SPF, DKIM, and DMARC authentication is set up.
Continuously monitor deliverability metrics such as open rates, click-throughs, and complaints.
Common pitfalls
Attempting to send an excessively high volume (e.g., 500k emails) on the first day of warm-up.
Using the ESP's domain instead of your own for sending, which divorces your brand from your sending reputation.
Failing to clean your email list before migration, leading to high bounce rates and spam traps.
Not monitoring deliverability metrics, missing early warning signs of reputation damage.
Expert tips
If major ISPs show reluctance to accept even a 50% daily increase, consider a safer 40% increase.
Ensure your campaign performance is closely monitored, especially for nonexistent complaints during warm-up.
Focus on content quality and relevance to encourage high engagement from early sends.
Be prepared to slow down or pause sending if you observe any negative deliverability trends.
Expert view
Expert from Email Geeks says that sending 500,000 emails on the first day of IP warm-up is far too much volume, and starting with as few as 500 emails on the first day and gradually increasing is a much safer approach.
2024-03-14 - Email Geeks
Expert view
Expert from Email Geeks says that the shared IP reputation is almost irrelevant in a B2C context, but you still need to warm up the new shared IP and domain combination with the new ESP.
2024-03-14 - Email Geeks
Navigating your ESP migration
Switching to a new ESP is an opportunity to optimize your email marketing, but the IP warm-up phase is where many missteps can occur. By understanding the importance of sender reputation and meticulously following best practices for gradual volume increases, list hygiene, and consistent monitoring, you can ensure a seamless transition.
Gmail, 
Outlook, and 
Yahoo). Without a proper warm-up, your emails risk being flagged as spam, ending up in the junk folder, or being rejected entirely. I've seen businesses inadvertently damage their deliverability by rushing this crucial phase.
