How do multiple or external domains in an email affect sender reputation and deliverability?
Matthew Whittaker
Co-founder & CTO, Suped
Published 1 Jul 2025
Updated 19 Aug 2025
7 min read
When you send an email, you might not think much about all the different domains it contains. Beyond your primary sending domain, there could be domains for images, tracking links, or content from third-party services. The question often arises, how do these multiple or external domains affect your sender reputation and, ultimately, your email deliverability?
It's a common misconception that simply including more than one domain automatically flags your email as suspicious. Billions of legitimate emails are sent daily with multiple domains mentioned in their body, and conversely, billions of spam emails contain only a single domain. The mere presence of multiple domains does not, in itself, mark a message as bad or a reason for it to be filtered.
The real impact on your sender reputation and deliverability comes down to the reputation of each individual domain mentioned within the email. If any of those domains, whether yours or an external one, carries a poor reputation, it can indeed lead to filtering or even blocking of your email, regardless of your primary domain's standing.
The basics of domain reputation
Understanding what constitutes domain reputation is fundamental. It's essentially a score that internet service providers (ISPs) assign to your domain based on your email sending behavior and, crucially, how recipients interact with your mail. This score influences whether your emails land in the inbox, spam folder, or are rejected outright. It's not just about your primary domain, but how email sending practices impact domain reputation as a whole.
Every domain you include in your email, including those for links, images, or even redirects, forms part of the overall reputation assessment. This is why having a strong, positive domain reputation is paramount. A positive reputation means higher inbox placement rates and greater trust from mailbox providers.
While your IP address has its own reputation, domain reputation often holds more weight. ISPs meticulously track how subscribers engage with emails from specific domains. This includes opens, clicks, replies, and crucially, spam complaints or deletions without opening. All these actions contribute to shaping the domain's reputation score. Learn more in our guide to understanding your email domain reputation.
It's important to remember that reputation isn't static. It changes day to day, or even hour to hour, depending on sending habits and recipient engagement. Therefore, consistent good sending practices are key to maintaining a healthy domain reputation.
Understanding the impact
While having multiple domains in an email isn't inherently problematic, the reputation of each domain is critical. If any included domain (e.g., for images, tracking, or external links) has a poor reputation, it can negatively affect your entire email's deliverability, even if your main sending domain is pristine.
How external domains impact your sending
When you include external domains in your emails, such as for third-party images, analytics tracking, or linked content, you are essentially asking mailbox providers to consider the reputation of those external domains alongside your own. If any of these external domains have a poor reputation, it can act as a red flag for spam filters, potentially leading to your email being routed to the spam folder. This is particularly true for deep links or redirected URLs.
Think of it this way: mailbox providers like Gmail (and others) assess every resource within an email collectively. This includes your sending domain, the IP address, all URLs (including those for images and tracking), and any other embedded content. The overall reputation of the entire message is derived from these combined elements. Therefore, a single poorly reputed external domain can bring down the entire message's score.
The key takeaway here is that you need to be mindful of all domains present in your email, not just the one in your From address. If a single domain with a sufficiently bad reputation is present, it can cause all mail containing it to be thrown into the bulk folder, irrespective of other factors.
Domain type
Impact on reputation
Example
Primary sending domain
Directly influences inbox placement based on sender behavior.
yourcompany.com
Tracking domains
Reputation of the tracking domain is factored in.
click.example.com
Image hosting domains
A compromised image host can hurt deliverability.
images.cdn.com
External link domains
Poor reputation can drag down your overall email score.
thirdpartysite.net
Mitigating risks and best practices
To protect your sender reputation and ensure high deliverability, proactive management of all domains within your emails is crucial. Here are some best practices to consider:
Vet third-party domains: Before linking to any external site or using third-party services for images or tracking, assess their reputation. A bad domain could lead to your emails going to spam.
Use subdomains strategically: If you send different types of emails (marketing, transactional, support), consider using separate subdomains for each. This can help compartmentalize your reputation so that issues with one type of send don't damage your entire domain's standing. Be aware of how domain reputation works with subdomains.
Maintain strong authentication: Implement and properly configure email authentication protocols like SPF, DKIM, and DMARC. These protocols help verify that your emails are legitimate and prevent spoofing, which in turn protects your domain's reputation. This is a key sender guideline from Google.
Monitor your domain reputation: Regularly check your domain's health using tools like Google Postmaster Tools. Pay attention to any unusual spikes in spam complaints or bounces, which can indicate issues with your content or list hygiene. Understanding how spam reports affect your reputation is vital.
Warm up new domains/IPs: If you're using a new domain or IP address, you'll need to warm it up gradually by sending small volumes of mail first. This helps establish a positive sending history with ISPs.
Even if both your primary sending domain and an external domain have independent good reputations, if you introduce them together on a new sending IP, that combination will need to establish its own reputation. This means a warm-up period might still be necessary to build trust with mailbox providers for that specific combination of resources.
Internal domains (your own)
Control: You have full control over the domain's configuration and sending practices.
Subdomain strategy: Use subdomains to segment different email streams (e.g., marketing.yourdomain.com) to insulate reputation.
Authentication: Ensure all internal domains and subdomains are properly authenticated with SPF, DKIM, and DMARC.
When to use
Always for your primary sender identity. Use subdomains for different types of mail, like newsletters versus transactional emails, to manage reputation more granularly.
External domains (third-party)
Vetting: Research the reputation of any linked external domains. Poor reputations can negatively affect your deliverability.
Content delivery networks (CDNs): Ensure images are hosted on reputable CDNs to avoid deliverability issues related to image domains.
When to be cautious
Whenever you lose direct control over a domain linked in your email, exercise caution. This includes links to partner websites, third-party content, or if you are changing ESPs and domains. Consider using your own branded tracking domains.
Conclusion
Ultimately, the presence of multiple or external domains in your emails is not inherently detrimental to your sender reputation or deliverability. The critical factor is the individual reputation of each domain involved.
By understanding how domain reputation works, diligently monitoring the health of all domains linked in your emails, and adhering to best practices like strong email authentication and strategic subdomain use, you can confidently navigate the complexities of email deliverability. Proactive management ensures that your messages reliably reach the inbox, maintaining your valuable sender reputation.
Views from the trenches
Best practices
Maintain consistent sending practices across all your domains and subdomains.
Use dedicated subdomains for different email types (e.g., marketing, transactional) to segment reputation.
Always ensure all domains in your emails are properly authenticated with SPF, DKIM, and DMARC.
Common pitfalls
Ignoring the reputation of external domains, such as image hosts or linked content, can negatively impact your primary domain.
Failing to warm up new domain/IP combinations, even if individual components have good reputations, can lead to filtering.
Assuming that simply having multiple domains in an email is automatically a spam indicator, which is often not the case.
Expert tips
Consider that mailbox providers evaluate all resources within an email holistically when determining its overall reputation.
Remember that user interaction with the mail is the primary driver of a domain's reputation, whether positive or negative.
Be aware that some ISPs might consider the combined reputation of multiple domains present in an email when making filtering decisions.
Expert view
Expert from Email Geeks says it is not the number of domains that matters, but rather the individual reputations of those domains.
2018-11-21 - Email Geeks
Expert view
Expert from Email Geeks says legitimate emails frequently contain multiple domains, while spam often contains only one.
Gmail (and others) assess every resource within an email collectively. This includes your sending domain, the IP address, all URLs (including those for images and tracking), and any other embedded content. The overall reputation of the entire message is derived from these combined elements. Therefore, a single poorly reputed external domain can bring down the entire message's score.
yourcompany.com
click.example.com
images.cdn.com
