Should I use a separate domain or subdomain for bulk email sending?
Matthew Whittaker
Co-founder & CTO, Suped
Published 4 Jul 2025
Updated 19 Aug 2025
9 min read
One of the most common questions I hear from businesses embarking on bulk email campaigns is whether to use their main domain, a subdomain, or even a completely separate domain for sending. This decision can significantly impact your email deliverability and overall sender reputation. There isn't a one-size-fits-all answer, as it depends on your specific sending volume, content type, and risk tolerance. However, industry best practices generally lean towards a particular approach.
Understanding how internet service providers (ISPs) evaluate email senders is crucial. They constantly monitor sending patterns, spam complaints, bounce rates, and engagement metrics to assign a sender reputation score to your domain and IP address. A high reputation means your emails are more likely to land in the inbox, while a low reputation can lead to messages being flagged as spam or even outright rejected. This is why the choice of domain for bulk sending is so important.
My goal is to help you navigate this decision by outlining the pros and cons of each method and providing clear guidance on how to maintain excellent deliverability, no matter your choice. We'll explore how different domain setups can impact your email program and the critical steps you need to take to protect your brand's reputation.
Understanding sender reputation
Sender reputation is the cornerstone of email deliverability. It's a score assigned by mailbox providers like Google and Yahoo to an IP address or domain based on its sending behavior. This score determines whether your emails reach the inbox, land in the spam folder, or are blocked entirely. Factors like spam complaints, bounces, unsolicited bulk email (UBE) messages, and even if your domain ends up on a blacklist or blocklist contribute to this reputation.
Your primary domain, such as yourcompany.com, typically hosts your website, transactional emails, and personal communications. Its reputation is paramount to your brand's overall online presence. If bulk email sending activities, especially marketing or cold outreach, negatively impact this domain's reputation, it can jeopardize critical business communications like password resets, order confirmations, and even one-to-one emails.
This is where the concept of segregating email streams comes in. By separating your bulk email sending from your core communications, you create a buffer zone. If something goes wrong with a marketing campaign (e.g., high spam complaints, an unexpected blocklist event), the damage is contained to the sending entity used for that specific stream, preserving the health of your primary domain. You can learn more about how to protect your main domain's reputation by segmenting email streams.
Understanding reputation tiers
Domain Reputation: This applies to the entire domain (e.g., yourdomain.com) and is influenced by all subdomains and IPs associated with it. A strong domain reputation signals legitimacy.
Subdomain Reputation: Each subdomain (e.g., marketing.yourdomain.com) builds its own reputation. This segregation is key for risk mitigation. You can read more about why to use subdomains for email marketing.
IP Reputation: The reputation of the specific IP address from which emails are sent. Shared IPs can sometimes affect your domain's reputation if other senders on that IP are problematic.
The subdomain advantage
For most businesses sending bulk emails (especially marketing or newsletter content), using a subdomain of your main domain is the recommended best practice. This approach allows you to leverage your existing brand recognition while isolating the reputation of your bulk email sending.
For example, if your main domain is yourcompany.com, you might use marketing.yourcompany.com or news.yourcompany.com. This clearly links the emails back to your brand, which builds trust with recipients, while also protecting your primary domain's reputation. The Messaging, Malware and Mobile Anti-Abuse Working Group (M3AAWG) provides detailed recommendations on this.
The main benefit of a subdomain is that it allows for granular reputation management. If your marketing emails encounter deliverability issues, the impact is primarily contained to the marketing subdomain. Your transactional emails and other crucial communications sent from your main domain remain unaffected, ensuring business continuity. This separation is particularly important for businesses sending over 5,000 emails per month, as per Google's and Yahoo's new sender requirements.
It is also simpler to set up authentication records (like SPF, DKIM, and DMARC) for subdomains. These records are essential for email security and help prove that you are an authorized sender, which further boosts your deliverability. Additionally, subdomains generally require a warm-up period, similar to new domains, but often benefit from the established reputation of the parent domain.
When a separate domain might be considered
While subdomains are generally preferred, there are specific scenarios where using a completely separate domain for bulk email sending might be considered. This is often debated in the cold outreach community. If your bulk sending activities are very high-risk, such as aggressive cold email campaigns with potentially low engagement or high bounce rates, a separate domain can provide an even greater layer of insulation from your primary brand.
However, using a separate domain comes with its own set of challenges. It lacks the immediate brand recognition that a subdomain offers, which can impact recipient trust and engagement. Furthermore, a new domain starts with no reputation, meaning it will require a significant warm-up process to build a positive sending history. This can be a time-consuming and delicate process. The Spamhaus FAQs on marketing emails emphasize that using too many unique domains can raise red flags for mail filters.
If you do opt for a separate domain, it's critical to choose a cousin domain that clearly relates to your primary brand. Avoid generic or completely unrelated domains, as these can appear suspicious to recipients and mail filters, potentially triggering phishing warnings. For example, if your company is Suped.com, you might choose SupedOutreach.com, but not EmailPros.net.
When a separate domain might be considered
High-risk campaigns: Cold outreach or lead generation campaigns with potentially lower engagement rates.
Strict brand protection: For organizations where any deliverability issue, no matter how small, is unacceptable for the main domain.
Specific IP/MTA needs: If you require completely isolated IP addresses and mail transfer agents (MTAs) that are distinct from your primary sending infrastructure.
Ultimately, the choice hinges on your risk assessment and the nature of your bulk emails. For typical marketing newsletters, a subdomain is almost always the safer and more practical choice. For highly aggressive or experimental sending, a separate domain might be considered, but only with a clear understanding of the added setup and reputation-building efforts required.
Best practices for any setup
Regardless of whether you choose a subdomain or a separate domain, several universal best practices are essential for maximizing email deliverability and ensuring your messages reach the inbox.
Authentication: Properly configure SPF, DKIM, and DMARC records. These protocols verify your sending identity and help prevent spoofing and phishing, which are major factors in deliverability. Failing to implement these correctly is a common reason emails fail to deliver.
List hygiene: Regularly clean your email lists to remove inactive subscribers, bounces, and spam traps. Sending to a clean, engaged list is vital for maintaining a good sender reputation. This is one of the most impactful ways to improve your domain reputation.
Content quality: Avoid spammy keywords, excessive images, broken links, or misleading subject lines. Deliver valuable, relevant content that recipients want to open and engage with. You can use an email deliverability tester to check your content before sending.
Monitoring your sender reputation is also critical. Regularly check your domain's health using tools like Outlook's Smart Network Data Services (SNDS) and Google's Postmaster Tools. Pay close attention to spam complaint rates and IP/domain reputation scores.
For ongoing vigilance, consider implementing a DMARC monitoring solution. DMARC reports provide invaluable insights into your email authentication results and identify potential deliverability issues early. You should also regularly check your sending domains against email blocklists (or blacklists) to ensure your sending reputation remains clean. By proactively managing these factors, you can maintain high deliverability, irrespective of your domain choice.
Views from the trenches
Best practices
Always use a subdomain for marketing, promotional, and bulk email streams to segregate reputation risks from your main domain.
Implement DMARC at a enforcement policy (p=quarantine or p=reject) as soon as possible on both your main domain and sending subdomains.
Monitor your domain and IP reputation regularly using Postmaster Tools and blocklist checking services.
Maintain meticulous list hygiene, removing inactive subscribers and hard bounces promptly to avoid spam traps.
Common pitfalls
Sending high-volume cold email campaigns from your main domain, risking its core reputation for transactional emails.
Neglecting email authentication (SPF, DKIM, DMARC) for new subdomains or separate domains, leading to deliverability issues.
Using a completely unrelated or suspicious-looking separate domain for bulk sending, which can trigger phishing warnings.
Ignoring spam complaints and bounce rates, which are key indicators of poor sender reputation and can lead to blocklisting.
Expert tips
Even with subdomains, poor sending practices can sometimes cascade and affect the main domain's reputation if issues are severe or prolonged.
For very high-risk sending like aggressive cold outreach, a separate domain might offer better isolation, but it requires significant warm-up.
Mail filters prefer stability, so consistent sending volume and content from a recognized domain (or subdomain) builds trust over time.
Internet presence of a domain (social media, website traffic) has minimal, if any, direct impact on email deliverability compared to sending metrics.
Expert view
Expert from Email Geeks says they never recommend using a separate domain for bulk mailing, as all best practices suggest using subdomains of the organizational domain.
2020-06-10 - Email Geeks
Expert view
Expert from Email Geeks says they have seen scenarios where a subdomain can affect organizational domain deliveries, but it only happens if there are serious issues. They also said that internet presence of a domain is not a significant factor in deliverability, as mail filters prioritize consistent content and stable campaign performance.
2020-06-10 - Email Geeks
Final thoughts on bulk email sending
For most companies engaged in legitimate bulk email activities, such as marketing newsletters, transactional emails, or promotional campaigns, using a dedicated subdomain is the clear best practice. It provides a crucial layer of protection for your primary domain's reputation while allowing you to maintain brand consistency.
While a separate domain might offer maximum isolation for extremely high-risk or experimental sending, the trade-offs in brand recognition and the effort required to build a new reputation often outweigh the benefits for most businesses. The key takeaway is consistent, responsible sending practices, rigorous list hygiene, and robust email authentication, regardless of your domain choice.
By following these guidelines, you can ensure your bulk emails reach their intended recipients, protect your brand, and foster positive relationships with your audience.
Google and 
Yahoo to an IP address or domain based on its sending behavior. This score determines whether your emails reach the inbox, land in the spam folder, or are blocked entirely. Factors like spam complaints, bounces, unsolicited bulk email (UBE) messages, and even if your domain ends up on a blacklist or blocklist contribute to this reputation.
Outlook's Smart Network Data Services (SNDS) and 
