What are the risks of sharing a sending domain with a partner and how does primary domain reputation affect others?
Matthew Whittaker
Co-founder & CTO, Suped
Published 28 May 2025
Updated 15 Aug 2025
7 min read
I've often seen businesses grapple with the complexities of email deliverability, especially when it involves partnerships and shared sending infrastructure. One common scenario that raises immediate red flags is when a business considers sharing its primary sending domain with a partner, or when a partner inadvertently uses a subdomain that links back to your main domain. This can lead to a tangled web of shared reputation and potential risks.
The core issue revolves around email domain reputation, a critical factor that internet service providers (ISPs) like Google and Microsoft use to determine whether your emails land in the inbox or the spam folder. When domains are shared, good or bad sending practices from one party can directly impact the deliverability of the other, creating significant challenges for email campaigns. I will explore the risks involved and how a primary domain's reputation affects others.
Understanding domain reputation
Domain reputation is essentially a trust score assigned to your sending domain by ISPs. It reflects your historical email sending behavior, including factors like spam complaints, bounce rates, subscriber engagement, and whether you've been listed on any email blocklists (or blacklists). A strong domain reputation means ISPs are more likely to trust your emails and place them in the recipient's inbox, while a poor reputation can lead to messages being filtered or rejected outright.
This reputation is continuously evaluated and updated based on your sending habits. Consistent, legitimate email practices build a positive score over time, which is why it’s vital to maintain a clean sender history. Conversely, sudden spikes in volume, high complaint rates, or sending to invalid addresses can quickly degrade your reputation.
I consider domain reputation the cornerstone of successful email deliverability. Without a good reputation, even perfectly crafted emails with valuable content may never reach their intended audience. It's the gatekeeper that determines your access to the inbox, making its maintenance a top priority for any sender.
The importance of good domain reputation
A good domain reputation ensures that your legitimate emails are delivered to the recipient's inbox, avoiding spam folders or outright rejection. This is crucial for maintaining effective communication with your audience and protecting your brand's credibility. It also provides a buffer against minor sending hiccups, as ISPs are more forgiving of domains they trust.
Sharing a sending domain with a partner, or even allowing them to use a subdomain of your primary domain for sending, introduces significant risks. The most immediate concern is the shared nature of reputation. If your partner engages in questionable sending practices, such as sending unsolicited bulk emails, experiencing high bounce rates, or generating numerous spam complaints, their actions will directly reflect on the shared domain's reputation. This shared negative impact can lead to your legitimate emails being blocked or landing in spam folders, even if your own sending practices are impeccable.
Another major risk is getting listed on email blacklists (or blocklists). If your partner's sending behavior triggers blacklist algorithms, the entire shared domain could be added, impacting all mail sent from it. Recovering from a blacklist can be a lengthy and challenging process, requiring significant effort to clean up your reputation and re-establish trust with ISPs. This scenario is a primary concern for email marketers.
I've also seen situations where a partner's email security (or lack thereof) can compromise the shared domain. If their systems are breached and used to send malicious emails, it reflects poorly on the domain, regardless of your own security measures. This highlights the importance of vetting partners thoroughly and having clear agreements regarding email sending policies and security protocols. For more on this, consider reading about the factors influencing reputation scores.
Dedicated sending domain
Reputation control: Your email sending reputation is solely your responsibility, making it easier to manage and protect.
Isolation of issues: Any deliverability problems or blacklistings are contained to your domain, not affecting others.
Branding: Reinforces your brand identity by using your own domain for all email communications.
Shared sending domain with a partner
Reputation risk: Your reputation is tied to your partner's sending practices, making you vulnerable to their poor behavior.
Deliverability impact: Partner's issues can lead to your emails being filtered to spam or blocked.
Blacklisting exposure: Higher chance of the shared domain ending up on a blacklist due to a partner's actions.
How primary domain reputation affects others
The primary (or root) domain's reputation casts a long shadow over its subdomains. While subdomains can build their own individual sending reputations, a severely damaged primary domain reputation can negatively impact its subdomains' deliverability, even if those subdomains have good sending practices. ISPs often consider the entire domain structure when evaluating trust.
This means that if your primary domain's reputation tanks due to your partner's actions (e.g., if they are sending from a subdomain of yours like marketing.yourdomain.com), it can negatively affect your ability to send emails from your main domain (e.g., yourdomain.com) for transactional or marketing purposes. The bad reputation can flow upwards, affecting the overall brand trust with mailbox providers. I've seen situations where this leads to significant deliverability challenges, requiring extensive reputation recovery efforts.
Risks of a shared primary domain
If you share your primary domain, or allow partners to use subdomains under your control, any poor sending practices by them (e.g., high spam complaints, sending to purchased lists) can directly impact your own email deliverability. This can result in your emails going to spam or being blocked by major ISPs like Outlook. It's a direct threat to your email program.
Ultimately, if a partner is somehow sending emails using your primary domain or a subdomain associated with it, you bear the ultimate responsibility for that sending reputation. This means having clear, enforceable agreements and maintaining strong control over your DNS records to prevent unauthorized or risky sending. If not managed properly, the primary domain's reputation will always be at risk.
Example SPF records for shared primary and subdomain sendingDNS
yourdomain.com. IN TXT "v=spf1 include:spf.sendgrid.net include:spf.partner.com ~all"
sub.yourdomain.com. IN TXT "v=spf1 include:spf.partneremail.com -all"
Mitigation strategies for shared sending environments
To mitigate the risks associated with shared sending domains, robust email authentication is non-negotiable. Implementing SPF (Sender Policy Framework), DKIM (DomainKeys Identified Mail), and DMARC (Domain-based Message Authentication, Reporting, and Conformance) on all sending domains and subdomains is crucial. This ensures that only authorized senders can use your domain, and it provides ISPs with a verifiable way to authenticate your emails, even if reputation is shared.
I also advise continuous DMARC monitoring and blocklist monitoring. DMARC reports provide invaluable insights into who is sending email using your domain, allowing you to quickly identify unauthorized sending or poor practices by partners. Blocklist monitoring helps you react swiftly if your domain or a related IP gets listed, minimizing downtime and reputational damage. This proactive approach is key to maintaining good standing.
Furthermore, establish clear and legally binding agreements with any partners regarding email sending policies. Define acceptable use, sending volumes, list hygiene requirements, and how to handle spam complaints or bounce rates. These agreements should also outline the consequences for non-compliance, ensuring both parties understand their responsibilities in protecting the shared domain's reputation. This is critical to preventing your emails from going to spam.
Measure
Description
Benefit
Email authentication
Set up SPF, DKIM, and DMARC for all sending domains and subdomains.
Verifies legitimate senders and helps prevent spoofing.
Identifies unauthorized sending and potential reputation issues early.
Clear partner agreements
Establish strict email sending policies and consequences for non-compliance.
Ensures mutual understanding and accountability for domain reputation.
Views from the trenches
I often discuss unique scenarios with others in the industry. Here are some insights from those conversations regarding shared domains and primary domain reputation:
Best practices
Implement robust email authentication protocols like SPF, DKIM, and DMARC across all your domains, especially when dealing with partners.
Establish clear, written agreements with partners outlining their responsibilities for email sending practices, list hygiene, and compliance.
Regularly monitor your DMARC reports and email blocklists to detect any unauthorized sending or sudden drops in reputation caused by partner activities.
If a partner insists on using a subdomain, ensure strict control over its DNS records and maintain oversight of their sending habits.
Segment your email sending by purpose using different subdomains (e.g., transactional, marketing) to isolate reputation and minimize risks.
Common pitfalls
Failing to implement DMARC in a monitoring mode (p=none) to gain visibility into all email sources using your domain, including those from partners.
Assuming a partner's email practices align with yours without proper due diligence, leading to unexpected reputation damage.
Ignoring DMARC reports, which can reveal unauthorized sending or misconfigurations that negatively impact your domain's trust score.
Not having a clear offboarding process for partners that includes revoking their access to your sending infrastructure.
Using a single shared sending domain for vastly different email streams (e.g., highly engaged transactional vs. cold outreach) without proper segmentation.
Expert tips
Always maintain ownership and control of your primary domain's DNS records, even when delegating subdomain management to partners.
Consider using a completely separate domain for high-risk or partner-managed email campaigns to fully isolate reputation.
Conduct periodic deliverability audits of your partner's sending practices to ensure ongoing compliance and prevent reputational issues.
Educate your partners on email best practices and the importance of maintaining a healthy sender reputation to foster a collaborative approach to deliverability.
Leverage DMARC policies at quarantine or reject (p=quarantine or p=reject) as soon as possible to prevent spoofing and protect your domain's reputation from abuse.
Marketer view
Marketer from Email Geeks says that if a competitor is using a subdomain of your main site, it's difficult to understand how that could happen without some form of DNS control or misunderstanding, making it hard to advise clearly.
2020-08-26 - Email Geeks
Marketer view
Marketer from Email Geeks says that if someone were to spam from a domain, it could theoretically be blacklisted, though reputation systems tend to rely more heavily on DKIM-signed domains, and a warm-up period would typically be required for new sending.
2020-08-26 - Email Geeks
Protecting your email reputation
In conclusion, sharing a sending domain with a partner, or having a primary domain that influences subdomains used by partners, presents significant email deliverability risks. The interconnected nature of domain reputation means that one party's poor sending practices can directly compromise the other's ability to reach the inbox, potentially leading to blacklisting and lost engagement. I've seen how challenging it can be to untangle these reputation issues once they arise.
To safeguard your email program and brand integrity, prioritize dedicated sending domains where possible. If sharing is unavoidable due to specific business needs, rigorously implement and monitor email authentication protocols like SPF, DKIM, and DMARC. Furthermore, establish crystal-clear agreements with partners that define acceptable sending behaviors and accountability. Proactive monitoring and robust policies are your best defense against the pitfalls of shared domain reputation.
Google and 
Microsoft use to determine whether your emails land in the inbox or the spam folder. When domains are shared, good or bad sending practices from one party can directly impact the deliverability of the other, creating significant challenges for email campaigns. I will explore the risks involved and how a primary domain's reputation affects others.
Outlook. It's a direct threat to your email program.
