Why are my emails going to spam after migrating to a new domain and ESP, and what steps can I take to improve inbox placement?
Michael Ko
Co-founder & CEO, Suped
Published 11 Jun 2025
Updated 17 Aug 2025
7 min read
Migrating your email sending to a new domain and Email Service Provider (ESP) is a significant undertaking. It is common to encounter challenges, especially when you start seeing your emails land in the spam folder. This can be particularly confusing if you are moving to a main domain you haven't extensively used for email before, or if you've been relying on cousin domains or subdomains previously.
When you switch, mailbox providers like Gmail, Outlook, and Hotmail often view your sending as a completely new identity. This means you have to re-establish your sender reputation from scratch, even if your domain has existed for a while. Poor inbox placement during this transition is not uncommon, but it requires careful diagnosis and a strategic approach to resolve.
Understanding the new sender identity
When you migrate to a new ESP, you are essentially changing your sending infrastructure. Even if you keep the same domain, the IP addresses used to send your emails are new, and mailbox providers need to learn to trust them. This process is called IP warming, and it is crucial. During warming, a low volume of emails is sent initially, gradually increasing over time. This helps build a positive sending history with ISPs.
Your domain's reputation, while partially portable, also experiences a reset of sorts. If you were sending from marketing.yourdomain.com and now you're sending from yourdomain.com for the first time, mailbox providers will treat it as a new sender because they haven't seen much email activity from that specific domain. This lack of sending history means they err on the side of caution, routing emails to spam until a positive engagement pattern is established.
The consistency of your sender identity is key. Even if your Spamhaus score looks good, indicating you're not on major blocklists, this doesn't guarantee inbox placement. ISPs use a much broader set of criteria, including engagement metrics and perceived trustworthiness, which are still being built for a new sending setup.
New ESP migration considerations
When switching to a new ESP like Brevo (formerly Sendinblue), it is crucial to understand how they manage shared or dedicated IP addresses and ensure proper warming is performed. Every ESP has its own practices for maintaining IP health, and yours needs to align with them.
Technical authentication and infrastructure
Before you even think about content, your email authentication records must be perfectly configured. These records tell receiving mail servers that your emails are legitimate and not spoofed. Improper setup is a primary reason for emails being flagged as spam, especially for a new sender.
SPF (Sender Policy Framework): Specifies which mail servers are authorized to send emails on behalf of your domain. If your new ESP's servers are not listed, emails will fail SPF checks.
DKIM (DomainKeys Identified Mail): Adds a digital signature to your outgoing emails, allowing the recipient's server to verify that the email was not altered in transit and truly came from your domain. Your new ESP will provide the DKIM keys you need to publish.
DMARC (Domain-based Message Authentication, Reporting, & Conformance): Builds on SPF and DKIM, telling receiving servers what to do with emails that fail authentication (e.g., quarantine, reject, or none). It also provides valuable reports on your email authentication status.
Even if your DMARC record is set to p=none, a lack of SPF and DKIM alignment can still impact deliverability. Mailbox providers increasingly demand strong authentication. Google and Yahoo's new requirements highlight the critical need for authentication for bulk senders.
Make sure your domain's DNS records for SPF, DKIM, and DMARC are correctly published and reflect your new ESP's sending infrastructure. Any errors here will cause immediate deliverability problems.
Content quality and recipient engagement
Even with perfect technical setup, the content of your emails plays a huge role in inbox placement. If your previous cousin domain emails were consistently hitting spam, it's possible the content itself (or the recipient engagement with it) was a factor. This problem can follow you to a new domain and ESP if not addressed.
Factors like using too many HTTP links instead of HTTPS, excessively long URLs, or large image files can raise red flags with spam filters. Filters scrutinize the email's HTML, looking for anything that might indicate suspicious activity. Using link shorteners like TinyURL can sometimes be counterproductive, as these are often abused by spammers and can lower your trust score.
Beyond technical content elements, consider the message itself. Is it highly promotional? Does it contain spammy keywords? Is the subject line misleading? While these might seem like basic marketing considerations, they directly impact deliverability. High complaint rates or low engagement (opens and clicks) on your old domain can signal to mailbox providers that your emails are not wanted, and this perception can influence how your new domain is initially treated.
Content pitfalls
Insecure links: Including http:// links in your email content instead of https:// can trigger spam filters.
Excessive images: Sending emails that are primarily images with minimal text is a common spammer tactic.
Spammy keywords: Certain words or phrases, often associated with promotions or scams, can land your emails in spam.
Content best practices
Use HTTPS: Ensure all links within your email content use https://.
Optimize images: Reduce image sizes using tools like TinyPNG and maintain a good text-to-image ratio.
Relevant content: Focus on providing value and relevance to your subscribers to boost engagement.
Warming up and reliable monitoring
Even if your email content is pristine and your authentication records are perfect, a new sending setup almost always requires a warming period. During this time, it is normal to experience temporary spam folder placement. Mailbox providers are observing how recipients interact with your emails to determine if you are a legitimate sender.
Tools that provide inbox placement testing (like GlockApps, which you are using) can offer directional data. However, their accuracy for specific B2B providers or during early warming stages can be limited. If your actual open and click rates on real campaigns are healthy, the test results might be skewed. Sometimes, even subject lines like test in test emails can trigger spam filters at Gmail.
Continuously monitor Google Postmaster Tools for your domain's spam rate and domain reputation. If your Postmaster spam rate remains consistently low (e.g., below 0.3%), this is a stronger indicator of health than fluctuating test results. Be patient; building a solid reputation on a new domain and ESP takes time and consistent positive sending behavior.
Strategic migration and ongoing vigilance
It is crucial to be methodical in your migration. Start with small, engaged segments of your audience and gradually increase volume. This strategy helps mailbox providers understand that your emails are wanted. A sudden influx of emails from a new domain or IP can overwhelm filters and lead to immediate blacklisting (or blocklisting).
Remember that deliverability is not a one-time fix, but an ongoing process. Even after migration, continuous monitoring of your sender reputation, email content, and engagement metrics is essential. By combining proper technical setup, careful warming, and diligent content practices, you can successfully navigate your migration and improve your inbox placement.
Views from the trenches
Best practices
Ensure your DNS records (SPF, DKIM, DMARC) are correctly configured for the new ESP before sending any emails.
Implement a structured IP and domain warming plan, starting with low volumes to highly engaged subscribers and gradually increasing.
Verify all links in your email content use HTTPS, and optimize image sizes to avoid triggering spam filters.
Regularly monitor your domain's reputation using tools like Google Postmaster Tools and your ESP's analytics.
Focus on sending highly relevant content to a well-segmented and engaged audience to build positive sending history.
Common pitfalls
Not performing proper IP or domain warming, leading to immediate high spam rates and potential blocklists.
Overlooking insecure HTTP links or large images in email content, which can be flagged by filters.
Relying solely on external inbox testing tools without cross-referencing with actual engagement metrics.
Ignoring potential content-related issues from previous sending habits that might carry over to the new domain.
Failing to update DNS authentication records (SPF, DKIM, DMARC) for the new email service provider.
Expert tips
Focus on the actual engagement metrics like opens and clicks from your real campaigns, not just seed list tests.
Understand that temporary spam folder placement is normal during the initial 2-3 days of domain warming.
Avoid using 'test' in subject lines for test emails sent to seed lists, as this can trigger spam filters.
If your corporate email is also seeing deliverability issues, investigate individual sender volumes within your organization.
Consider that content issues are often carried over during migrations, especially if problems persist despite new infrastructure.
Expert view
Expert from Email Geeks says that if your corporate email is on the parent domain, IT should look into the volumes of individual senders, especially sales, to see if anyone is sending over 100 messages per day.
December 5, 2024 - Email Geeks
Expert view
Expert from Email Geeks says to check the HTML file of your email content for any insecure 'http:' links and the sizes of images, as combined size can impact deliverability.
Gmail, 
Outlook, and 
Hotmail often view your sending as a completely new identity. This means you have to re-establish your sender reputation from scratch, even if your domain has existed for a while. Poor inbox placement during this transition is not uncommon, but it requires careful diagnosis and a strategic approach to resolve.
Brevo (formerly Sendinblue), it is crucial to understand how they manage shared or dedicated IP addresses and ensure proper warming is performed. Every ESP has its own practices for maintaining IP health, and yours needs to align with them.
Google and 
Yahoo's new requirements highlight the critical need for authentication for bulk senders.
TinyPNG and maintain a good text-to-image ratio.
Google Postmaster Tools for your domain's spam rate and domain reputation. If your Postmaster spam rate remains consistently low (e.g., below 0.3%), this is a stronger indicator of health than fluctuating test results. Be patient; building a solid reputation on a new domain and ESP takes time and consistent positive sending behavior.
