What are best practices for warming up a new subdomain for email sending, and how does it impact DKIM alignment?
Michael Ko
Co-founder & CEO, Suped
Published 2 Jun 2025
Updated 17 Aug 2025
9 min read
Warming up a new subdomain for email sending is a critical step in establishing a positive sender reputation. It's essentially a process of gradually increasing your email volume over time, allowing mailbox providers (like Google and Yahoo) to observe your sending behavior. This careful ramp-up helps them trust that your mail is legitimate and not spam. Without proper warming, a new subdomain might immediately encounter deliverability issues, with emails landing in spam folders or being rejected outright.
The process isn't just about volume, though, it's also about building a consistent sending pattern and ensuring your technical setup, especially DKIM alignment, is flawless. This foundational work ensures your emails reach the inbox effectively and helps you avoid getting placed on an email blocklist (or blacklist) from the outset.
What is subdomain warming and why is it essential?
Subdomain warming is the controlled process of sending emails from a new subdomain, starting with low volumes to highly engaged recipients and gradually increasing both volume and audience breadth. This practice helps build a positive sender reputation with internet service providers (ISPs) and mailbox providers, who track sending patterns and engagement metrics to determine trustworthiness. A sudden burst of email from a new, unknown sending domain or IP address can trigger spam filters, leading to poor inbox placement or even outright rejection.
The rationale behind using a subdomain rather than your main domain for marketing or transactional emails is to isolate its reputation. If your marketing emails encounter deliverability issues, using a dedicated subdomain (e.g., marketing.yourdomain.com) protects the reputation of your primary domain (e.g., yourdomain.com), which you might use for core business communications. This isolation is crucial for maintaining overall email program health. For more detailed insights on this process, explore our guide on switching to a new subdomain for email.
The actual timeline for warming a new subdomain varies based on factors like the volume you plan to send, the quality of your recipient list, and your content. There isn't a one-size-fits-all schedule, but the principle remains the same: start small and slowly scale up. Rushing this process is a common pitfall that can lead to deliverability issues, as it doesn't give mailbox providers enough time to assess your sending legitimacy. Patience and consistent monitoring are key to successful subdomain warming.
Ensuring proper authentication for your new subdomain
Email authentication protocols such as SPF, DKIM, and DMARC are crucial for verifying that emails are legitimate and come from an authorized sender. For a new subdomain, correctly configuring these records is not just a best practice, it is a requirement to achieve inbox placement. Google and Yahoo have recently tightened their sender requirements, making authentication non-negotiable for bulk senders. You can learn more about these fundamental protocols in our simple guide to DMARC, SPF, and DKIM.
DKIM (DomainKeys Identified Mail) alignment is particularly relevant when using a new subdomain. DKIM works by allowing the sender to digitally sign outgoing emails, providing a way for recipient servers to verify that the email has not been altered in transit and that it genuinely originated from the claimed domain. The crucial part for subdomains is how this signature aligns with the 'From' header domain. There are two types of alignment: relaxed and strict.
With relaxed alignment, the DKIM signing domain (d= tag) can be a subdomain of the 'From' header domain. For example, if your 'From' header is info@yourdomain.com, and your DKIM signature is for s1.emails.yourdomain.com, this will pass DMARC alignment under a relaxed policy because emails.yourdomain.com is a subdomain of yourdomain.com. Most common email configurations use relaxed alignment. Strict alignment, on the other hand, demands an exact match between the 'From' header domain and the DKIM signing domain, meaning a subdomain would cause a failure. While rare for DMARC policies to demand strict alignment, it is worth checking if you have DMARC enabled on your main domain. For more details on this, you can check our guide on changing an email's return-path domain.
DKIM record example
This is a typical DKIM record for a subdomain. Replace selector1 with your actual selector and yoursubdomain.yourdomain.com with your sending subdomain. The public key (p=) will be provided by your email service provider (ESP).
Example DKIM Record for Subdomain AuthenticationDNS
selector1._domainkey.yoursubdomain.yourdomain.com. IN TXT "v=DKIM1; k=rsa; p=MIGfMA0GCSqGSIb3DQEBAQUAA4GNADCBiQKBgQDIdyU/kR5R/w5T/g+V8g9W8n2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Y2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2YZ2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2Z2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Best practices for a successful warm-up schedule
The core of any subdomain warm-up is a gradual increase in sending volume. Begin by sending small batches of emails to your most engaged subscribers. These are recipients who frequently open, click, or reply to your emails, demonstrating a high level of interest. Their positive interactions will signal to mailbox providers that your emails are valued, helping to build trust from the ground up. Over time, you'll expand to less engaged segments of your list.
Consistency is paramount throughout the warming process. It's better to send smaller volumes daily than large volumes sporadically. Mailbox providers value predictable sending patterns. Also, ensure your content is high-quality, relevant, and free of spammy triggers. Even during warming, your content directly impacts how recipients interact with your emails and, consequently, your subdomain's reputation. We have more detailed advice on how to warm up a subdomain for email sending.
While Microsoft (Outlook/Hotmail) offers pre-emptive accommodations or whitelisting for new IP addresses to help with deliverability, this rarely applies to new subdomains. Mailbox providers generally focus on IP reputation first, then domain reputation. While there's no harm in submitting a ticket to Outlook.com for your subdomain, it's unlikely to have a significant impact compared to a proper warm-up schedule. The effectiveness of any warm-up is highly dependent on your specific content and audience engagement. For more insights on warming practices for these major providers, refer to our guide on warming up new email domains with Yahoo and Microsoft.
Increase by 50-100% daily (expand to slightly less engaged)
8-14
Continue gradual increase, introduce more segments
15+
Monitor closely, scale as deliverability allows
Monitoring reputation and troubleshooting deliverability
Throughout the warm-up process, consistent monitoring of your sender reputation and deliverability metrics is essential. Pay close attention to engagement rates (opens, clicks), complaint rates, and bounce rates. Tools like Google Postmaster Tools provide valuable insights into your domain's reputation with Google. Low engagement or high complaint rates are red flags that could indicate you're increasing volume too quickly or your content isn't resonating with your audience.
If you encounter deliverability issues during warm-up, such as emails landing in spam or being rejected, pause your sending and analyze the problem. Check your DMARC reports for authentication failures and review any bounce messages for clues. It's also wise to check if your subdomain has inadvertently landed on any email blocklists (or blacklists). Address any underlying issues before resuming the warm-up, and consider scaling back your volume temporarily. This proactive approach helps prevent minor setbacks from becoming major deliverability crises.
A critical piece of advice often overlooked is to resolve any existing deliverability problems on your main domain or other sending channels before starting a new subdomain warm-up. Migrating to a new subdomain won't magically fix pre-existing issues related to list quality, content, or authentication. In fact, attempting to do so can transfer those problems to your new subdomain, making remediation even more challenging. A clean slate starts with resolving current problems, ensuring your new subdomain has the best chance to build a pristine reputation. Our guide on why your emails fail offers solutions for common deliverability issues.
Views from the trenches
Best practices
Start by identifying your most engaged recipients to seed your initial sending volume.
Ensure all authentication protocols, SPF, DKIM, and DMARC, are correctly configured.
Maintain consistent daily sending volumes, avoiding large fluctuations, even on weekends.
Monitor key metrics like open rates, click-through rates, and complaint rates closely.
Sign 'old' mail with the new subdomain's DKIM signature to establish early association.
Prioritize sending high-quality, relevant content to minimize spam complaints.
Common pitfalls
Attempting to warm up a subdomain without first resolving existing deliverability issues.
Increasing sending volume too quickly, leading to immediate spam flagging.
Neglecting to set up or verify all necessary email authentication records.
Failing to monitor engagement and reputation metrics throughout the warm-up.
Sending inconsistent volumes or pausing warm-up for extended periods, especially weekends.
Mixing promotional and transactional emails during the early stages of warm-up.
Expert tips
Always target your most active users first when warming a new sending domain.
Set up a DMARC policy with relaxed alignment to ensure compatibility with subdomains.
Remember that warm-up schedules depend heavily on your content and audience specifics.
Don't expect Microsoft's pre-emptive accommodations to apply to subdomains; they are primarily for IPs.
Consider isolating different email streams (e.g., marketing, transactional) on separate subdomains.
Be prepared to slow down or pause sending if you observe negative trends in your deliverability data.
Expert view
Expert from Email Geeks says that when warming a new subdomain, it's beneficial to sign existing mail with the new subdomain's DKIM signature to establish an association and introduce the subdomain to the mail stream.
March 29th, 2024 - Email Geeks
Marketer view
Marketer from Email Geeks asked if using the main domain in the From header and a new subdomain for DKIM would break alignment.
June 18th, 2024 - Email Geeks
Building a strong sender reputation
Warming up a new subdomain is a foundational element of effective email deliverability. It's a strategic process that involves more than just sending emails, it requires careful planning, meticulous technical setup, and continuous monitoring. By adhering to best practices, such as starting with highly engaged segments, gradually increasing volume, and ensuring all authentication protocols like DKIM are correctly configured for relaxed alignment, you lay the groundwork for a robust sender reputation.
Remember, the goal is to systematically demonstrate to mailbox providers that you are a legitimate sender sending desired email. Patience and vigilance are your greatest allies in this process. Addressing any existing deliverability issues before you begin, and maintaining a focus on list quality and relevant content, will significantly contribute to your subdomain's long-term success in reaching the inbox rather than the spam folder.
Microsoft (Outlook/Hotmail) offers pre-emptive accommodations or whitelisting for new IP addresses to help with deliverability, this rarely applies to new subdomains. Mailbox providers generally focus on IP reputation first, then domain reputation. While there's no harm in submitting a ticket to 
Outlook.com for your subdomain, it's unlikely to have a significant impact compared to a proper warm-up schedule. The effectiveness of any warm-up is highly dependent on your specific content and audience engagement. For more insights on warming practices for these major providers, refer to our guide on warming up new email domains with Yahoo and Microsoft.
