When warming up new IPs, is it better to warm up per domain or per MX?
Michael Ko
Co-founder & CEO, Suped
Published 21 Apr 2025
Updated 18 Aug 2025
8 min read
When you're launching a new IP address for email sending, one of the most critical steps is the warm-up process. This gradual increase in sending volume helps build a positive sender reputation with mailbox providers (ISPs). A common question arises, however: is it better to warm up your new IPs based on individual domains or based on the Mail Exchanger (MX) records, which might cover multiple domains under one provider?
The distinction between warming up per domain versus per MX matters because ISPs evaluate your sending behavior to assign a reputation score. This score dictates whether your emails land in the inbox, spam folder, or are rejected entirely. Understanding how different warming strategies interact with these reputation systems is crucial for maximizing your deliverability.
Understanding IP vs. domain reputation
Sender reputation is a multifaceted concept that ISPs use to determine the trustworthiness of incoming email. While IP reputation has historically been a primary factor, domain reputation (the reputation of your sending domain) has gained increasing importance over time. Major providers like Gmail and Yahoo heavily weigh domain reputation when filtering emails.
An IP warm-up helps establish the credibility of your new IP address by sending a steadily increasing volume of mail to engaged recipients. This teaches ISPs that your IP is sending legitimate, desired mail, not spam. Similarly, a domain warm-up builds trust for your specific sending domain. You can learn more about this process in our guide on warming up an IP address for email sending.
MX records, on the other hand, point to the mail servers responsible for accepting email on behalf of a domain. Multiple domains might share the same MX record if they are hosted by the same email service provider. For instance, Outlook.com, Hotmail.com, and Live.com all share Microsoft's mail servers. So, the core of the question lies in which level of aggregation ISPs (and specifically their filters) primarily evaluate for reputation.
The case for warming per domain
For most senders, particularly those who manage a limited number of sending domains, warming up per domain is often the more straightforward and effective approach. This method provides finer control over the reputation of each individual domain. Since domain reputation is increasingly important, focusing your warm-up efforts directly on each domain's sending behavior makes intuitive sense. This strategy is also more manageable for many email teams.
Warming up per domain means you apply your progressive sending schedule to each specific domain (e.g., yourdomain.com, news.yourdomain.com). This allows ISPs to evaluate the traffic quality associated with each domain directly. If one domain experiences issues, it's less likely to negatively impact the reputation of your other domains, offering a degree of isolation. This method is generally recommended when you are warming up a new sending domain on an existing IP.
When migrating to a new platform or setting up a completely new sending infrastructure, a comprehensive warm-up strategy is always necessary for both IP and domain. As noted by Resend, this progressive increase in sending volume helps to maximize deliverability, especially when changing sending patterns or moving to new IPs. For more general information on this, you can refer to Resend's domain and IP warm-up guide.
Per domain warm-up
Control: More granular control over each domain's reputation.
Risk isolation: Issues with one domain (e.g., spam complaints) are less likely to affect others.
Simplicity: Generally easier to manage and track for most senders.
The case for warming per MX
While warming up per domain is often preferred, warming up per MX can be a viable strategy in specific scenarios, particularly for high-volume senders or those managing a very large portfolio of domains that resolve to the same MX. The argument for MX-based warming centers on the idea that ISPs might aggregate reputation at the MX level, especially for large mailbox providers that host numerous domains under unified mail infrastructure.
If you are sending to multiple domains that share the same MX record (like yahoo.de, yahoo.fr, and yahoo.it all using mx-eu.mail.am0.yahoodns.net), sending volume to that MX address could contribute to a broader reputation for all domains it serves. This might seem more efficient for very large-scale operations. However, this approach offers less control and can be harder to troubleshoot if deliverability issues arise for a specific domain within that MX group. For more on building reputation, consider this article on IP warm-up.
The complexity of warming up per MX lies in accurately identifying all domains under a particular MX and then distributing your send volume appropriately across them. This requires sophisticated infrastructure and meticulous tracking to ensure a balanced warm-up. In most cases, the additional effort involved may not yield significantly better results compared to a well-executed per-domain warm-up. However, if you're dealing with a massive volume of emails across many related domains and have the technical capabilities, it might be worth exploring. Our guide on dedicated IP warm-up best practices can offer additional insights.
Per domain warm-up
When warming up per domain, you gradually increase your sending volume for each specific domain you use to send emails. This builds a distinct reputation for each domain, which is crucial as ISPs increasingly focus on domain-level trust. This approach offers clear visibility into each domain's performance and makes it easier to pinpoint and resolve any issues.
Focus: Builds specific domain reputation.
Visibility: Easier to monitor individual domain performance and troubleshoot.
Common use: Ideal for most senders with a manageable number of domains.
Per MX warm-up
Warming up per MX involves increasing sending volume to the Mail Exchanger record, which may serve multiple domains. The idea is that reputation might be aggregated at this server level. While theoretically efficient for very large email infrastructures with many domains sharing common MX records, this method can be complex to implement and manage effectively.
Complexity: Requires advanced infrastructure and tracking.
Control: Less granular control over individual domain reputations.
Common use: Niche use for high-volume senders with unified MX infrastructure.
Practical considerations for warming
Regardless of whether you choose to warm up per domain or per MX, the core principles of IP warming remain the same. The goal is to build a positive sending reputation by gradually increasing your volume, maintaining high engagement rates, and minimizing complaints, bounces, and spam trap hits. Consistency is key, and a well-planned warm-up schedule is essential.
Monitoring your sender reputation during the warm-up period is crucial. Utilize tools like Google Postmaster Tools to track your IP and domain reputation, spam complaint rates, and deliverability metrics. Keep a close eye on any increases in bounces or spam placements, as these are indicators that you might be warming up too aggressively or encountering issues with your list quality.
The length of your warm-up period can vary, typically ranging from a few weeks to over a month, depending on your sending volume and the responsiveness of your audience. Some estimates suggest it can take 30-45 days to fully warm a set of IPs. Patience is a virtue here, as rushing the process can severely damage your new IP and domain reputation, potentially leading to immediate blocklisting (or blacklisting).
For transactional emails, where deliverability is paramount, warming up new IPs should still follow best practices to avoid service interruptions. Even if your initial email list is small, dedicating time to a proper warm-up can prevent issues down the line. We have more insights on warming up new IPs for transactional emails as well.
Here's an example of a simple warm-up schedule by domain, assuming a 30-day period. This schedule can be adjusted based on your specific sending volume and engagement rates.
Example daily warm-up schedule per domainplaintext
Day 1-3: 100-200 emails per domain (highly engaged recipients)
Day 4-7: 300-500 emails per domain
Day 8-14: 700-1,000 emails per domain
Day 15-21: 1,500-2,500 emails per domain
Day 22-30: 3,000-5,000+ emails per domain, or target full volume
This schedule is a general guideline. Key factors that influence the duration and speed of warming up new IP addresses include the size and quality of your contact list, the type of emails you're sending (transactional vs. marketing), and the responsiveness of your recipients. A highly engaged list allows for a faster warm-up.
Views from the trenches
Best practices
Start with your most engaged subscribers to generate positive interactions early in the warm-up.
Segment your audience by ISP (Internet Service Provider) and gradually increase volume to each separately.
Continuously monitor your sender reputation and deliverability metrics using available tools.
Send consistent volumes daily rather than sporadic bursts to build a steady reputation.
Ensure proper email authentication: SPF, DKIM, and DMARC are correctly configured.
Common pitfalls
Sending too much volume too quickly, leading to spam folder placement or blocklisting (blacklisting).
Sending to unengaged or old lists, resulting in high bounce rates and spam complaints.
Ignoring feedback loops or DMARC reports, missing critical signs of deliverability issues.
Failing to segment lists by engagement, pushing high volumes to uninterested recipients.
Assuming that one warm-up schedule fits all ISPs; some require more gradual ramping.
Expert tips
Prioritize domain reputation, as many ISPs now weigh it heavily alongside IP reputation.
If warming per MX, ensure robust infrastructure to distribute volume evenly across domains.
Consider a longer warm-up period for highly sensitive industries or problematic recipient lists.
Use a dedicated IP when possible for better control over your sending reputation.
Never send cold emails from a new, unwarmed IP or domain.
Marketer view
A Marketer from Email Geeks says that while warming up per MX theoretically seems better, per-domain warming has proven effective enough in practice, so whatever works is fine.
2020-03-05 - Email Geeks
Marketer view
A Marketer from Email Geeks says that warming per MX requires significantly more effort to do well, and for establishing a rough reputation curve, warming per domain is likely sufficient.
2020-03-05 - Email Geeks
Prioritising your warm-up approach
Ultimately, for most senders, warming up new IPs per domain offers better control, clearer visibility, and a more manageable process for building a solid sender reputation. While warming per MX might be a consideration for extremely high-volume senders with specific infrastructure setups, it introduces added complexity without guaranteeing significantly superior results for typical email operations.
Focus on the fundamental principles of gradual volume increase, positive engagement, and diligent monitoring. By adhering to these best practices, you can effectively warm up your new IPs and domains, ensuring your emails reach the inbox reliably. If you're encountering deliverability challenges, our guides on why emails go to spam can provide further assistance.
Gmail and 
Yahoo heavily weigh domain reputation when filtering emails.
Outlook.com, 
Hotmail.com, and 
Live.com all share 
Microsoft's mail servers. So, the core of the question lies in which level of aggregation ISPs (and specifically their filters) primarily evaluate for reputation.
