What are the deliverability risks of setting up a sender domain on a different ESP?
Matthew Whittaker
Co-founder & CTO, Suped
Published 30 Jun 2025
Updated 16 Aug 2025
7 min read
When a company acquires another, or an organization expands its digital footprint, a common question arises: Can we set up a sender domain for one entity on the Email Service Provider (ESP) of another? For example, using company1.com as a sending domain through Company 2’s ESP. While this might seem straightforward, it introduces several complexities and potential deliverability risks that need careful consideration.
The core concern revolves around email deliverability and sender reputation. Your sender reputation is a critical factor that mailbox providers (like Google and Yahoo) use to decide whether your emails land in the inbox, spam folder, or are blocked entirely. This reputation is tied to both your sending IP address and your domain.
My initial thought often leans towards minimal impact if the sender domain is unique and uses separate IPs. However, the reality is more nuanced. Even with distinct infrastructure, the relationship between the domains and the overall sending practices of each company can create unexpected deliverability challenges.
Sender reputation and domain types
When you introduce a new sender domain on a different ESP, it essentially starts with a neutral reputation. Mailbox providers have no historical data on this domain's sending patterns, engagement, or complaint rates. This blank slate can lead to initial skepticism from filtering systems, which are designed to be cautious of new or unfamiliar senders.
The way a domain is perceived by internet service providers (ISPs) can be influenced by whether it's a completely new, unrelated domain (sometimes referred to as a cousin domain) or a subdomain of an existing, reputable domain. A cousin domain, like company1-email.com when your main brand is company1.com, can be highly suspicious to filters, often being treated as a phishing attempt or a malicious spoof. This can directly hurt your sender reputation.
Conversely, a subdomain such as email.company1.com offers a stronger link to the established brand. This allows the new sending domain to inherit some of the existing goodwill and trust associated with the main domain, provided the primary domain has a good sending history. This approach is generally more favorable for deliverability.
Regardless of the domain type, if the new sending domain utilizes a fresh set of IP addresses from Company 2’s ESP, these IPs also need to build their own reputation. This process is known as IP warm-up, and it's essential for preventing deliverability issues. Without it, even legitimate emails can be sent to the spam folder or blocked entirely.
Authentication and technical setup
Cousin domain
A cousin domain (e.g., company1-email.com) is separate from your primary brand domain. It offers no inherent reputation transfer from your main brand and starts from scratch.
Reputation: No reputation inherited, must build trust independently.
Suspicion: Often flagged as suspicious, especially by major mailbox providers like Microsoft and Gmail.
Risk: Higher risk of emails landing in spam or being outright blocked. Mailbox providers such as M3AAWG strongly discourage their use due to security and confusion risks.
Subdomain
A subdomain (e.g., email.company1.com) is a part of your primary brand domain. It signals a clear association with your established brand identity.
Reputation: Can inherit some reputation from the root domain, giving it a head start.
Credibility: Perceived as more legitimate and trustworthy by recipient servers.
Recommendation: Generally the recommended approach for new sending domains to leverage existing brand trust.
For either domain type, ensuring proper email authentication is non-negotiable. This involves configuring SPF, DKIM, and DMARC records correctly within the new ESP’s system and your DNS. These records verify that your emails are legitimate and sent by authorized servers, preventing them from being flagged as spam or phishing attempts.
Misconfigured or missing authentication records are a major red flag for mailbox providers. They can lead to significant deliverability problems, regardless of your sender reputation. It's crucial that the DNS records for the new sender domain are correctly aligned with the ESP’s sending infrastructure to ensure emails pass authentication checks. If your previous ESP used a different setup, you'll need to carefully migrate these configurations to the new provider.
Example SPF record
An SPF record is a TXT record in your DNS that specifies which mail servers are authorized to send email on behalf of your domain. Here's a typical example:
SPF TXT RecordDNS
v=spf1 include:spf.example-esp.com ~all
This record tells receiving servers that emails from your domain are only valid if they originate from spf.example-esp.com. It's important to consult your specific ESP for their required SPF record to avoid issues.
The critical importance of warm-up
The warm-up process is arguably the most critical step when setting up a new sender domain on a different ESP, especially if you're also using new IP addresses. Mailbox providers are inherently cautious of new sending domains or IPs, as these are often used by spammers. To build trust, you must gradually increase your sending volume over time, sending to highly engaged recipients first.
This gradual increase allows mailbox providers to observe positive sending behavior, such as low complaint rates, high open rates, and minimal bounces. Rushing the warm-up process by sending high volumes too quickly can trigger spam filters, leading to immediate blocklisting (or blacklisting) or messages landing directly in the spam folder. Building a strong sender reputation requires consistent, positive engagement.
The warm-up period can take anywhere from a few weeks to several months, depending on your sending volume and list quality. For comprehensive guidance on this, consider resources like this Spamhaus guide on changing providers. It's crucial to send only to engaged recipients during this phase to build a positive sending history. This also applies when sending cold emails from a new setup.
Key warm-up considerations
Gradual increase: Start with low volumes and slowly scale up daily.
Engaged recipients: Send to your most active subscribers initially.
Consistency: Maintain consistent sending volumes and frequencies during warm-up.
Monitoring: Actively monitor deliverability metrics and sender reputation tools.
Indirect impacts and ongoing monitoring
Even with a new sender domain and separate IP addresses, there can be indirect ways that one company's sending reputation might affect the other. If the emails sent from the new domain on Company 2’s ESP contain links back to Company 1’s main website or refer to the original brand name, issues with the new domain could potentially reflect poorly on the primary brand.
For instance, if the new sender domain generates a high volume of spam complaints or gets added to a major blocklist (or blacklist), some mailbox providers might make a connection to the parent brand, especially if shared content, branding, or linked domains exist. While direct impacts are minimized with separate infrastructure, brand association can still play a role in how your emails are filtered.
Continuous monitoring of your sender reputation for both domains is essential. Tools that track blocklist status, complaint rates, and engagement metrics are invaluable. DMARC reports provide critical insights into authentication failures and potential abuse, helping you proactively address issues. Keeping an eye on these signals allows you to respond quickly if deliverability issues arise with the new sender domain, minimizing potential harm to your overall brand reputation. You can also monitor your domain reputation in Google Postmaster Tools.
Views from the trenches
Best practices
Always use a subdomain (e.g., email.yourcompany.com) rather than a cousin domain for new sending setups to leverage existing brand trust.
Implement a thorough email warm-up process for any new sending domain and its associated IPs, even if on a different ESP.
Ensure all email authentication records (SPF, DKIM, DMARC) are correctly configured and aligned for the new sender domain within the new ESP.
Segment your audience and start warming up new sending infrastructure by sending to your most engaged subscribers first.
Regularly monitor deliverability metrics, including bounce rates, spam complaints, and blocklist status, for both the new and existing domains.
Common pitfalls
Using cousin domains (e.g., yourcompany-mail.com) that appear similar but are separate can trigger spam filters and damage brand trust.
Skipping or rushing the email warm-up process, leading to immediate spam folder placement and negative sender reputation.
Failing to properly configure SPF, DKIM, and DMARC for the new domain, resulting in authentication failures and delivery issues.
Sending to unengaged or old lists from a new domain, which generates high bounces and complaints, harming reputation.
Not monitoring the performance of the new sending domain, allowing deliverability issues to fester and potentially impact brand reputation.
Expert tips
Start with very low sending volumes to highly engaged segments. Gradually increase volume and expand to less engaged segments only once initial volumes show good inbox placement.
Focus on content quality and relevance from day one with the new domain. High engagement will significantly accelerate positive reputation building.
Set up DMARC reporting for the new domain. Analyze reports daily during warm-up to identify any authentication or delivery issues early.
If the primary domain has a poor reputation, a new, completely unrelated domain might be considered after thorough analysis, but it's a higher risk strategy.
Remember that mailbox providers value consistency. Maintain a stable sending volume and content strategy once warm-up is complete.
Expert view
Expert from Email Geeks says that there's no simple answer as it depends on many factors, and abstract advice without concrete details can be misleading. However, cousin domains are generally not recommended.
2023-11-23 - Email Geeks
Expert view
Expert from Email Geeks says that if a company's main domain has a poor reputation, sending messages from a new domain that mentions or links to it could cause problems, even if the new domain is on separate infrastructure.
2023-11-23 - Email Geeks
Navigating your sender domain strategy
Setting up a sender domain on a different ESP, particularly after an acquisition or expansion, involves navigating several deliverability risks. While using separate IP addresses and a unique sender domain minimizes direct impact on the original company’s reputation, it doesn't eliminate all challenges.
The key to success lies in meticulous planning and execution. Opting for a subdomain of the primary brand, ensuring all authentication records are correctly configured, and strictly adhering to a comprehensive warm-up schedule are paramount. Continuous monitoring of key metrics will help you identify and resolve issues swiftly, protecting both the new and established sender reputations.
By taking these precautions, you can effectively mitigate the deliverability risks and ensure that emails sent from the new domain successfully reach their intended inboxes, supporting your overall communication strategy without compromising brand trust.
Google and 
Yahoo) use to decide whether your emails land in the inbox, spam folder, or are blocked entirely. This reputation is tied to both your sending IP address and your domain.
Microsoft and 
Gmail.
