How should I target mailbox providers during an email warmup?
Michael Ko
Co-founder & CEO, Suped
Published 18 May 2025
Updated 17 Aug 2025
6 min read
Starting a new email sending journey, whether with a fresh IP address or a new domain, requires a crucial first step: warming up your email infrastructure. This process isn't just about sending a few emails and hoping for the best; it's about systematically building a positive sender reputation with mailbox providers (also known as Internet Service Providers or ISPs). Without a proper warm-up, your legitimate emails risk being flagged as spam or outright blocked, severely impacting your deliverability.
A common question arises during this phase: how should I target mailbox providers during an email warmup? The answer is nuanced, as different providers have varying criteria for judging sender reputation, and their market share can differ significantly by region. A one-size-fits-all approach often leads to suboptimal results.
My goal is to guide you through a strategic approach to warming up your sending infrastructure by effectively targeting key mailbox providers, ensuring your emails reach the inbox.
The necessity of targeted warming
Every mailbox provider, such as Gmail, Outlook, and Yahoo, monitors sender behavior to protect their users from unwanted mail. They track metrics like spam complaints, bounces, open rates, click-through rates, and whether recipients move messages from spam to the inbox. Consistent positive engagement signals to them that you are a legitimate sender. Neglecting this crucial phase can lead to your emails being directed to the spam folder or even blocked.
The key during the warm-up process is to demonstrate to these providers that your recipients genuinely want to receive your emails. This involves sending to engaged segments of your list who are most likely to open, click, and reply. Starting with smaller volumes and gradually increasing them over time allows mailbox providers to recognize your sending patterns and build trust.
Effective warm-up isn't just about avoiding the spam folder, it's about building a strong sender reputation that ensures consistent inbox placement in the long run. Think of it like building credit history: you start small and prove your reliability before being granted more trust. This principle applies equally to IP warm-up and domain warm-up.
Identifying your audience's mailbox providers
Before you can target mailbox providers, you need to understand which ones are most prevalent in your existing email list. This is a critical first step. Most email service providers (ESPs) allow you to segment your lists by domain. By analyzing your subscriber data, you can identify the distribution of domains like gmail.com, outlook.com, yahoo.com, aol.com, and various corporate domains.
Once you have this breakdown, you can create targeted segments for your warm-up sends. The most effective strategy involves sending to your most engaged recipients first. These are subscribers who have recently opened or clicked your emails, as they are highly likely to interact positively, which is exactly what mailbox providers look for during a warm-up period. By focusing on these engaged users, you signal positive behavior to the ISPs right from the start.
General warm-up
Approach: Sends a mix of emails to various domains without specific focus on provider distribution.
Reputation building: Less efficient, as it doesn't prioritize providers where you need to build the most trust.
Risk: May trigger spam filters or blocklists if volume is disproportionate to provider expectations.
Targeted warm-up
Approach: Segments by domain, focusing on the largest providers and most engaged users first.
Reputation building: Highly effective, as it tailors sending volume and engagement signals to each provider's needs.
Benefit: Improves inbox placement and reduces the likelihood of being blocklisted.
Geographic breakdown of key providers
The major mailbox providers can vary significantly by region. While Gmail and Outlook (Hotmail/Live) are dominant globally, other providers can have substantial market share in specific countries or regions. For instance, you might see higher volumes for iCloud or Comcast in North America, while European regions may have strong local providers. This diversity underscores the importance of segmenting your warm-up strategy by geographic region if your audience is geographically dispersed.
To effectively tailor your warm-up, it helps to know the general landscape of mailbox providers by region. While your actual list data should always be your primary guide, this overview provides a starting point for understanding typical distribution. Identifying these dominant providers allows you to prioritize your efforts and allocate sending volume accordingly, ensuring that you build a robust reputation where it matters most for your specific audience. You can often find email domain lists with associated providers for general research, such as this GitHub project.
Once you have identified your major mailbox providers and their regional distribution, you can craft a targeted warm-up plan. This plan involves gradually increasing your sending volume to each specific provider, ensuring that your ramp-up schedule aligns with their tolerance levels. It's not just about total volume, but also about the volume directed to each major player.
Start by sending to your most engaged subscribers across all major providers. As your engagement metrics (opens, clicks, low complaints) remain strong, gradually increase the volume sent to each specific provider. Pay close attention to providers like Gmail and Microsoft, as they often have the strictest filtering. If you notice a drop in inbox placement or an increase in bounces for a particular provider, slow down your sending to that provider and assess your content and list quality.
Key warm-up considerations
Content consistency: Use consistent content that matches your typical sending style and purpose.
Actively monitoring your deliverability metrics throughout the warm-up process is non-negotiable. Look for trends in inbox placement, open rates, and bounces for each specific mailbox provider. Tools that provide insights into your domain and IP reputation, as well as those that monitor blocklists (or blacklists), are invaluable. If you see signs of trouble, such as a sudden increase in spam folder placement or being listed on a blacklist, pause your sending and investigate immediately. Adjust your volume or content as needed to regain trust.
Views from the trenches
Best practices
Always start your warm-up by sending to your most recently engaged and active subscribers.
Segment your email list by domain to accurately track and manage sending volumes to each major mailbox provider.
Ensure all email authentication (SPF, DKIM, DMARC) is correctly configured before starting your warm-up.
Increase your sending volume gradually and consistently, avoiding large spikes that could look suspicious.
Monitor key deliverability metrics, like open rates and complaint rates, for each provider during the warm-up.
Common pitfalls
Sending to unengaged or old email addresses during the initial warm-up phase, which can trigger spam traps and complaints.
Ignoring the specific requirements or nuances of different mailbox providers like Gmail, Outlook, or Yahoo.
Increasing sending volume too quickly, leading to emails being sent to the spam folder or IP blacklisting.
Not monitoring deliverability metrics closely enough, missing early signs of issues with specific providers.
Using generic, untargeted email content that doesn't encourage positive engagement from recipients.
Expert tips
If your client has existing data, analyze it to determine the breakdown of mailbox providers and their geographic distribution.
Prioritize sending to domains that represent the largest portion of your engaged audience first.
Focus on consistent interaction and positive engagement signals, as this is what truly builds sender reputation.
Consider that major mailbox providers vary by region, so tailor your strategy if you have a global audience.
Leverage the insights from your email service provider regarding recommended warm-up schedules for different volumes.
Expert view
Expert from Email Geeks says a warm-up should generally use segments of an existing opted-in email list rather than generic emails to made-up accounts, which would not be effective.
October 2, 2024 - Email Geeks
Marketer view
Marketer from Email Geeks says it is best to take a relative sample of your opt-in lists and send slow and low, with gradual growth over several weeks.
October 2, 2024 - Email Geeks
Achieving inbox success
Targeting mailbox providers effectively during an email warm-up is not just a best practice, it's a fundamental requirement for successful email deliverability. By understanding the unique characteristics of your audience's dominant providers, segmenting your lists intelligently, and meticulously monitoring your sending performance, you can build a strong sender reputation that ensures your emails consistently land in the inbox.
Gmail, 
Outlook, and 
Yahoo, monitors sender behavior to protect their users from unwanted mail. They track metrics like spam complaints, bounces, open rates, click-through rates, and whether recipients move messages from spam to the inbox. Consistent positive engagement signals to them that you are a legitimate sender. Neglecting this crucial phase can lead to your emails being directed to the spam folder or even blocked.
iCloud or 
Comcast in North America, while European regions may have strong local providers. This diversity underscores the importance of segmenting your warm-up strategy by geographic region if your audience is geographically dispersed.
