What are common email deliverability issues during new IP and subdomain warmup after ESP migration?
Matthew Whittaker
Co-founder & CTO, Suped
Published 27 Jul 2025
Updated 19 Aug 2025
6 min read
Migrating to a new Email Service Provider (ESP) is a significant undertaking, often accompanied by the need to warm up new IP addresses and subdomains. This process is crucial because mailbox providers, like Google and Microsoft, are highly cautious of traffic from unknown sources to prevent spam.
Even when following recommended warm-up procedures, such as gradually increasing sending volume to engaged users, we commonly encounter unexpected deliverability issues. These problems can range from emails landing in spam folders to temporary blocks, even if our email content is legitimate and our subscriber acquisition is above board.
Understanding these common pitfalls is vital for a smooth transition and to protect your sender reputation. We will explore the key challenges faced during new IP and subdomain warm-up after an ESP migration and provide insights into resolving them.
The initial hurdles of new infrastructure
When you start sending from a new IP address or a new subdomain, these identifiers have no prior sending history, meaning they also have no established reputation. Mailbox providers interpret this lack of history as a potential risk. They operate on a principle of trust, and new senders haven't yet earned that trust.
This suspicion often leads to initial throttling, where your emails are intentionally slowed down, or, more concerningly, direct placement into the spam folder without explicit spam complaints from recipients. You might notice this through lower open rates and users contacting you because they haven't received your emails. Monitoring your reputation using tools like Google Postmaster Tools and Outlook.com Postmaster is essential during this phase.
In some cases, if the sending patterns are too aggressive or suspicious, your IP or subdomain might even end up on a public or private blocklist (also known as a blacklist). Being listed can severely impact your ability to reach the inbox, leading to outright rejections. We have seen this happen even with legitimate senders simply because the volume increase was too rapid for a nascent reputation.
Why new IPs are suspicious
Mailbox providers treat new IP addresses and subdomains with extreme caution primarily because a significant volume of unsolicited spam originates from unknown or newly established sending infrastructure. This means that even if your intentions are legitimate, your new sending setup is initially seen as a potential threat.
Without an established history of positive engagement and adherence to sending best practices, ISPs are quick to apply stricter filtering rules. This can result in emails being redirected to the spam folder, delayed, or even blocked entirely. This protective measure is in place to safeguard their users from malicious or unwanted content.
Authentication and technical misconfigurations
A common oversight during ESP migration is the improper configuration of email authentication protocols like SPF, DKIM, and DMARC. These records are fundamental for proving that your emails are legitimate and that you are authorized to send from your domain. A missing or incorrect SPF record, for instance, can immediately flag your emails as suspicious, regardless of your content or volume.
For example, if you've delegated your sending domain to a previous ESP and are now warming up a new subdomain and IP with a new provider, ensuring all DNS records are correctly updated for the new infrastructure is paramount. A missing SPF record means recipient servers cannot verify that the new IP is authorized to send on behalf of your domain, leading to immediate distrust and potential blocks.
Example SPF recordDNS
v=spf1 include:_spf.newesp.com ~all
Similarly, DKIM (DomainKeys Identified Mail) provides a digital signature, ensuring the email hasn't been tampered with in transit. Without proper DKIM setup, your emails lose another layer of trust. DMARC (Domain-based Message Authentication, Reporting, and Conformance) builds on SPF and DKIM, allowing you to set policies for unauthenticated emails and receive reports on their status. Always prioritize setting up these authentication records from day one to establish your sender identity.
Correctly configured authentication
SPF record: Includes your new ESP's sending IPs or domains, correctly identifying authorized senders.
DKIM signature: Properly signed emails verify integrity and sender authenticity with a valid key.
DMARC policy: Monitors authentication results and informs mailbox providers how to handle unauthenticated mail.
These configurations build immediate trust, signaling to mailbox providers that your emails are legitimate and from an authorized source.
Missing or incorrect authentication
Missing SPF: Recipient servers cannot verify the sender, leading to spam folder placement or rejection.
Incorrect DKIM: Emails appear tampered with or unsigned, undermining trust and reputation.
No DMARC: Missed opportunity to gain insights into authentication failures and enforce policies.
Lack of proper authentication significantly damages your sender reputation, leading to poor deliverability.
Content quality and audience engagement
Beyond technical setup, the actual content of your emails and how recipients interact with them play a huge role during warm-up. Even with legitimate opt-ins, certain content, like image-heavy daily deal emails, can be perceived as spammy by mailbox providers' filters. If emails are going to spam without complaints, it often points to content or engagement issues rather than direct blocklisting.
During warm-up, it is crucial to send only to your most engaged segments. High open rates and click-through rates from these recipients signal positive intent to ISPs. Conversely, low engagement, even from a small subset of recipients, can severely hinder your warm-up progress. It’s a quality-over-quantity game during this sensitive period.
If you're noticing good open rates overall but still struggling with specific providers like Gmail or Hotmail, it could be related to their unique filtering algorithms or specific content triggers. Sometimes, even subtle elements in your email design or wording can cause issues. Continual monitoring and A/B testing your content can help identify and mitigate these problems.
Metric
Why it matters during warmup
Goal during warmup
Open rate
Indicates recipient engagement and interest, crucial for building sender trust.
High (e.g., 20%+ depending on industry)
Click-through rate (CTR)
Shows deeper engagement and content relevance, further enhancing reputation.
Consistent and healthy (e.g., 2-5%+)
Spam complaint rate
Direct negative feedback signaling recipient dissatisfaction.
As low as possible (below 0.1%)
Bounce rate
Indicates issues with recipient addresses (hard bounces) or temporary server issues (soft bounces).
Low (under 2% for hard bounces)
Inbox placement
The ultimate measure of deliverability success – reaching the primary inbox.
Consistently high for target ISPs (90% or more)
Managing volume and expectations
One of the most tempting, yet detrimental, actions during warm-up is to drastically increase sending volume too quickly. Mailbox providers expect a gradual, consistent increase in email volume over several weeks. Jumping from small test sends to hundreds of thousands of emails, even to highly engaged users, can trigger volume filters and lead to throttling or blocks, negating all warm-up efforts.
It is crucial to set realistic expectations for the warm-up period. This isn't a flick-of-a-switch operation. Depending on your sending volume and list quality, a full warm-up can take weeks or even months. Persistence and careful monitoring are key. If you face issues with a specific ISP, like Hotmail, remember that some providers (especially those with tabbed inboxes) might inherently have different deliverability outcomes, even on well-warmed IPs.
Patience and persistence are key
Warming up a new IP address or subdomain is a marathon, not a sprint. Rushing the process by sending too much volume too quickly can severely damage your new sender reputation, leading to long-term deliverability issues like being frequently blocklisted or directed to the spam folder. Building trust with mailbox providers takes time and consistent positive engagement signals.
Even with perfect practices, some ISPs may be more conservative, requiring extra patience. The goal is to gradually prove your legitimacy, allowing them to confidently place your emails in the inbox. Consistent monitoring of your metrics and adapting your strategy based on performance are far more effective than aggressive volume increases.
Views from the trenches
Best practices
Maintain strict adherence to your warm-up schedule, gradually increasing volume over several weeks, not days.
Send to your most engaged segments first during the initial warm-up phase to generate positive engagement signals.
Implement all email authentication protocols - SPF, DKIM, and DMARC - before sending from new infrastructure.
Monitor your deliverability metrics closely using postmaster tools and analytics to catch issues early and adapt.
Common pitfalls
Sending too much volume too quickly, overwhelming mailbox providers and triggering spam filters.
Neglecting email authentication setup for new IPs or subdomains, which instantly damages sender trust.
Ignoring feedback from postmaster tools, missing critical insights into reputation decline or spam placement.
Continuing to send to unengaged or old segments during warmup, leading to low engagement and spam complaints.
Expert tips
Patience is paramount: It is crucial to understand that a full warm-up can take weeks, and sometimes months.
Content matters: Even with legitimate sign-ups, 'spammy' content can lead to spam folder placement.
Consistency: Maintaining a consistent sending volume and pattern helps establish and maintain sender reputation.
Delegation: If your old ESP controls your domain, plan carefully how to regain control without disrupting sends.
Marketer view
Marketer from Email Geeks says they are encountering difficulties with Hotmail deliverability during their current warm-up process.
2017-09-04 - Email Geeks
Marketer view
Marketer from Email Geeks says the new tabbed inbox in Hotmail might be contributing to deliverability issues, suggesting that senders might not achieve the same rates as with old IPs.
2017-09-04 - Email Geeks
Navigating your ESP migration for smooth deliverability
Migrating to a new ESP and warming up new IP addresses and subdomains is a challenging but necessary process for maintaining strong email deliverability. The key to success lies in a meticulous approach that addresses potential issues proactively. Ensuring proper email authentication (SPF, DKIM, DMARC), carefully managing sending volume, and focusing on high engagement from your most active subscribers are critical steps.
Continuous monitoring of your sender reputation and deliverability metrics through postmaster tools and other analytics will provide the insights needed to adapt your strategy. While temporary dips in inbox placement can be frustrating, patience and adherence to best practices will ultimately lead to a strong and trusted sending reputation on your new infrastructure.
