How do open rates and send rate throttling behave during a new subdomain warmup?
Matthew Whittaker
Co-founder & CTO, Suped
Published 29 May 2025
Updated 19 Aug 2025
8 min read
Warming up a new subdomain for email sending is a critical phase for your email deliverability. It's akin to building trust with internet service providers (ISPs) and mailbox providers, showing them that your sending practices are legitimate and your emails are valued by recipients. This process requires patience and careful monitoring, as both open rates and send rate throttling can behave in unexpected ways during this period.
The goal of a subdomain warmup is to gradually increase your sending volume while maintaining high engagement and low complaint rates. This teaches ISPs that your new subdomain is a source of desired mail, rather than spam. Ignoring this crucial step can lead to significant deliverability issues, including emails landing in spam folders, or worse, being rejected outright.
Many factors influence how your subdomain performs during this period, from the quality of your contact list to the content of your emails and your email authentication setup. Understanding the nuances of open rate fluctuations and how to manage send rate throttling is key to a successful warmup and long-term email program health.
Open rate dynamics during warmup
It's quite common to observe open rates exhibiting an initial peak followed by a stabilization, or even a slight decline, during the course of a new subdomain warmup. This pattern often occurs because, during the early stages, you typically send to your most engaged segments, such as newly onboarded users who have recently interacted with your product or service. These recipients are highly likely to open and interact with your emails, artificially inflating early open rates.
As the warmup progresses and you gradually increase your sending volume, you inevitably begin to include a broader segment of your audience, some of whom may have lower engagement tendencies. This expansion naturally brings the overall open rate down to a more realistic average for your typical campaigns. A Reddit discussion highlights how a business saw a significant drop in open rates after a domain change, struggling to maintain previous levels, which is a common experience during this transition.
While a decline from an initial high might seem concerning, it's not necessarily a negative sign, provided the rates remain within an acceptable range for your industry and email type. What's crucial is to maintain consistent positive engagement. This means focusing on metrics beyond just opens, such as click-through rates, reply rates (especially for B2B outreach), and low complaint rates. These deeper engagement signals tell ISPs that your mail is valued, helping to build a solid sender reputation for your new subdomain.
If you notice an abrupt drop or consistently low open rates, it could indicate an underlying issue, such as poor audience targeting, content that isn't resonating, or even being mistakenly identified as spam. Monitoring your domain reputation through tools like Google Postmaster Tools is essential for gaining insight into how mailbox providers perceive your sending. This helps you understand if you need to adjust your strategy for a healthier subdomain warmup.
Navigating send rate throttling
Send rate throttling refers to the practice of limiting the number of emails sent per hour or per day to a specific mailbox provider. ISPs (Internet Service Providers) like Gmail and Outlook implement throttling to prevent sudden floods of mail from new or unknown sending sources, which could indicate spam. During a new subdomain warmup, adhering to a strict, gradual increase in sending volume and hourly throttling is paramount. My experience shows that if you don't warm IPs (or subdomains) appropriately, your email delivery speed could be significantly throttled or slowed.
The key is to avoid triggering the ISPs' spam filters or security protocols. A sudden surge in volume from a new subdomain can be seen as suspicious behavior. For instance, Google's sender guidelines emphasize gradual increases to help ensure messages are delivered as expected and prevent rate limiting. The decision to ease up on throttling isn't a one-size-fits-all, blanket approach, but rather a dynamic process tailored to each mailbox provider.
You can begin to ease throttling when you consistently receive positive SMTP responses and observe no signs of pushback, such as temporary failures or deferrals, from a specific mailbox provider. This granular approach allows for optimal and faster warmup. If you encounter temporary failures (often called temp fails), it's a clear signal to reduce your sending rate to that particular provider. Ignoring these signals could lead to a blocklist (or blacklist) listing.
For significant volumes, such as an eventual 180,000 emails per day, aggressive throttling at the start is crucial, especially when your audience is largely composed of free domains, which are often considered lower quality due to higher spam rates. Once you establish a solid reputation with a particular ISP, you can progressively increase the volume. Generally, doubling your volume weekly after the initial cautious phase is a common goal for building trust.
Key factors influencing a successful warmup
A successful subdomain warmup hinges on several interconnected factors that collectively build your sender reputation. While open rates and throttling are direct indicators, the underlying elements are equally important.
Audience quality: Begin your warmup by sending to your most engaged and responsive contacts. These are individuals who have recently interacted with your brand or explicitly opted-in to receive your emails. This strategy helps generate strong positive signals, such as opens, clicks, and replies, which are vital for building a positive reputation with ISPs.
Content relevance: Ensure your email content is highly relevant, valuable, and expected by your recipients. Avoid anything that could be perceived as spammy or unsolicited. Plain text emails for onboarding or transactional purposes are often excellent choices for warmup, as they typically elicit high engagement and are less likely to trigger spam filters than highly promotional content.
Email authentication: Proper configuration of SPF, DKIM, and DMARC is non-negotiable. These protocols prove to ISPs that your emails are legitimately coming from your domain, preventing them from being flagged as phishing or spoofing attempts. Errors in these configurations can severely hinder your warmup efforts, regardless of your sending volume or engagement rates.
Continuously monitoring key metrics, such as bounce rates, spam complaint rates, and unsubscribe rates, alongside your open and click rates, provides a holistic view of your sender reputation. A high complaint rate, even with good opens, can quickly land your subdomain on a blocklist or blacklist, undoing your warmup progress.
Optimizing your subdomain warmup
Optimizing your subdomain warmup involves a strategic approach to volume, audience selection, and ongoing monitoring. For a detailed guide on the process, consider how to warm up a new subdomain effectively.
When you're ready to switch triggered campaigns to your new subdomain, do so gradually. Start with low-volume, highly engaged campaigns, and monitor their performance closely before migrating higher-volume flows. This phased approach allows you to detect and address any issues early without jeopardizing your overall email program.
Send rate best practices
Gradual increase: Increase daily volume slowly, typically by 10-20% increments, and monitor how ISPs respond. The Postmark guide on warming up a domain is a good reference.
Provider-specific throttling: Adjust your throttling based on the responses from individual mailbox providers, not a general rule. This requires careful observation of SMTP logs and bounce messages.
Engagement focus: Prioritize sending to highly engaged users initially to build positive sender reputation signals during the warmup period.
Open rate management
Expect fluctuations: Initial high open rates are normal due to engaged audiences, and a subsequent decrease to a stable, realistic level is also expected as volume increases.
Monitor broader metrics: Look at click-through rates, reply rates, and spam complaint rates. These indicate genuine interest and deliverability success.
Audience segmentation: Gradually introduce less engaged segments into your sending pool as your subdomain reputation strengthens.
Understanding and adapting to how open rates and send rate throttling behave during a new subdomain warmup is essential for establishing a robust email sending infrastructure. While initial open rate spikes and subsequent stabilization are normal, continuous monitoring of all engagement metrics is vital. Your approach to throttling should be dynamic, responding to feedback from individual mailbox providers rather than a rigid schedule.
By focusing on high-quality audience segments, maintaining relevant content, and ensuring proper email authentication, you lay the groundwork for strong sender reputation. This meticulous attention to detail during the warmup phase will pay dividends, ensuring your emails consistently reach the inbox and drive the desired engagement.
Views from the trenches
Best practices
Start warm-up with your most engaged subscribers who are most likely to open and click.
Gradually increase your sending volume to individual mailbox providers based on their feedback.
Monitor SMTP response codes to identify any throttling or temporary failures per provider.
Ensure SPF, DKIM, and DMARC are correctly configured for your new subdomain.
Focus on maintaining low bounce and spam complaint rates throughout the warmup.
Use Google Postmaster Tools and other monitoring dashboards for insight into your reputation.
Common pitfalls
Increasing send volume too quickly, leading to throttling or blocklisting.
Ignoring specific mailbox provider feedback (e.g., SMTP deferrals).
Sending to unengaged or old lists during the warmup phase.
Neglecting email authentication records, which signals untrustworthy sending.
Panicking over initial open rate fluctuations without looking at other metrics.
Not having a clear plan for gradually migrating triggered campaigns to the new subdomain.
Expert tips
A gradual ramp-up is crucial for any new sending subdomain.
Don't solely focus on open rates. Prioritize clicks, replies, and low complaint rates.
Consider segmenting your audience by mailbox provider during warmup to fine-tune throttling.
Regularly check blocklists and blocklist monitoring services.
Prepare a rollback plan in case of severe deliverability issues.
Warmup isn't just about volume, it's about building trust and consistent engagement.
Expert view
Expert from Email Geeks says that if you start sending to your highest engagers, metrics will gradually decline as you introduce more lower engagement recipients.
2024-11-08 - Email Geeks
Expert view
Expert from Email Geeks says that send rate is not something across the board but should be managed per mailbox provider. You can titrate the rate up if SMTP responses show no problems, and dial it down for a specific vendor if you get pushback.
Gmail and 
Outlook implement throttling to prevent sudden floods of mail from new or unknown sending sources, which could indicate spam. During a new subdomain warmup, adhering to a strict, gradual increase in sending volume and hourly throttling is paramount. My experience shows that if you don't warm IPs (or subdomains) appropriately, your email delivery speed could be significantly throttled or slowed.
