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 14 Aug 2025
10 min read
Summary
The presence of multiple or external domains within an email does not inherently harm sender reputation or deliverability. Instead, the critical factor is the individual reputation of each domain mentioned or linked within the message. Internet Service Providers (ISPs), particularly major ones like Gmail, assess all resources (domains, URLs, links, images, IP addresses) within an email collectively to determine its overall reputation.
A domain with a sufficiently poor reputation can negatively impact the deliverability of an entire email, even if other elements in the message have a good standing. Conversely, good user engagement with emails containing multiple domains can lead to a positive combined reputation for that specific email sending pattern. Therefore, while multi-domain emails are common and legitimate, maintaining a strong reputation for all included domains is crucial.
Key findings
Multiple domains: Simply having multiple domains in an email body is not a direct trigger for spam filters. Billions of legitimate emails contain more than one domain.
Reputation is key: Deliverability issues arise when one or more of the domains referenced in an email (including link URLs) have a poor reputation, regardless of other elements.
Holistic assessment: ISPs like Gmail evaluate all resources within a message—domains, URLs, images, and IP addresses—together to form a combined reputation for that specific email or sending pattern.
User interaction: Domain reputation is largely built on how recipients interact with emails containing that domain. Positive interactions (opens, clicks) improve it, while negative ones (spam complaints, deletions) degrade it.
Overriding bad reputation: A severely bad reputation for a single included domain can cause an entire email to be filtered to the spam folder, even if the primary sending domain has a good reputation.
Key considerations
Domain and IP warmup: When using a new domain on an unfamiliar IP address, a warmup process is necessary to build a combined reputation, even if both entities have independent good reputations.
Monitoring all domains: Senders must monitor the reputation of all domains included in their emails, not just their primary sending domain. Tools like Google Postmaster Tools can help with this.
Content and link quality: Regularly auditing all linked URLs for their reputation is essential. A single problematic link can taint an otherwise clean email (see e-shot's insights on deep links).
Subdomain strategy: For different types of email (transactional, marketing), using separate subdomains can help compartmentalize sending reputation, preventing issues with one stream from affecting another. More on this in our guide to email sending domain best practices.
What email marketers say
Email marketers often express concern about whether including multiple domains, particularly external ones, can negatively impact their email campaigns. While it's a valid concern, the consensus among marketers aligns with expert opinions: the mere presence of multiple domains is usually not the problem. The core issue lies with the reputation of *those specific domains*. Marketers emphasize the importance of vigilant monitoring of all linked domains and the holistic view ISPs take when assessing email trustworthiness.
Key opinions
Not quantity, but quality: Many marketers believe that the number of domains isn't the issue, but rather the reputation of each individual domain included in the email.
Linked URLs matter: Link URLs, especially those from third parties (e.g., tracking domains, external content), are a significant factor. If they have a poor reputation, they can drag down the email's overall score.
ISP-specific variations: It's acknowledged that different ISPs might weigh domain reputation differently, leading to varied impacts across providers.
Combined reputation: The email as a whole develops a reputation based on all its components (sending domain, linked domains, IP), influenced heavily by user engagement.
Aligning domains: While not strictly necessary to avoid deliverability issues, aligning all domains (from headers to links) is often considered a best practice for consistency and trust signals.
Key considerations
Vigilant monitoring: Marketers should continuously monitor the reputation of all domains used in their emails, especially those they don't directly control. This includes their primary sending domain, tracking domains, and any domains linked in content (a concept highlighted by Encharge on email reputation).
Sender authentication: Proper implementation of authentication protocols like SPF, DKIM, and DMARC for all sending and linked domains is critical to signal legitimacy and trust to ISPs. Our guide to DMARC, SPF, and DKIM provides more detail.
Engagement optimization: Focusing on high recipient engagement (opens, clicks, replies) is paramount, as this positive feedback helps build a strong reputation for the entire email, including any linked domains.
Marketer view
An email marketer from Email Geeks explains that the issue isn't typically the presence of multiple domains within an email, but rather the individual reputation of each domain. If any of the domains, whether primary or external, have a poor standing, this is what triggers deliverability problems.
22 Nov 2018 - Email Geeks
Marketer view
A marketer from Quora clarifies that domain reputation is paramount. They suggest that unless a domain has a very poor reputation and is on blacklists, its presence alone won't drastically impact deliverability. The critical factor is the established trust or distrust associated with the domain.
22 May 2024 - Quora
What the experts say
Deliverability experts consistently highlight that the mere inclusion of multiple domains in an email does not automatically trigger spam filters or reduce sender reputation. Their core message is that the reputation of the *specific domains themselves* is the deciding factor. They emphasize that ISPs evaluate the entire email as a collective unit, and user interaction (positive or negative) with that specific email determines its unique reputation, which can be heavily influenced by any low-reputation domains it contains.
Key opinions
Neutral factor: Experts agree that the presence of multiple domains is not inherently a negative marker. Billions of legitimate emails daily contain various domains without issues.
Reputation, not count: The key is whether any of the domains mentioned (e.g., in links) have a poor reputation. A bad domain reputation can indeed lead to email filtering.
Holistic assessment: ISPs, particularly Gmail, assess all resources (domains, URLs, links, images, IPs) within an email together to form its overall reputation.
User interaction defines reputation: Reputation is fundamentally about how recipients interact with the mail. If emails containing certain URLs are consistently marked unwanted, that URL (and by extension, the email) develops a bad reputation.
Combined reputation takes precedence: An email with a mix of good and bad reputation domains will develop its own unique reputation based on recipient feedback for that specific message content.
Key considerations
Impact of poor reputation: If a domain within an email has a sufficiently poor reputation, it can cause the entire message to be sent to the spam or bulk folder, regardless of the primary sending domain's reputation. This highlights why what happens when your domain is on a blocklist is critical to understand.
Domain and IP warmup for new combinations: Even if a domain and an IP address individually have good reputations, using them together for the first time requires a warmup period to establish a new combined reputation.
Removing problematic domains: In extreme cases, removing a domain with a very poor reputation from an email's body can significantly improve deliverability, as demonstrated by practical experience. This reinforces the importance of knowing how to recover domain reputation.
Consistency with senders: While aligning domains isn't a hard rule, maintaining uniformity in email headers and content (where practical) contributes to a cohesive and trustworthy sending profile, as noted by some experts.
Expert view
A deliverability expert from Email Geeks explains that having multiple domains within an email is not the problem; instead, it is when some of those domains possess poor reputations that issues arise.
22 Nov 2018 - Email Geeks
Expert view
A deliverability expert from Spam Resource emphasizes that mailbox providers meticulously calculate domain reputation, which directly influences inbox placement. Factors like email list segmentation and sending practices are crucial in this assessment.
22 Mar 2024 - Spam Resource
What the documentation says
Official documentation from various email service providers and industry bodies consistently underscores the importance of domain reputation as a critical factor in email deliverability. While they don't explicitly forbid the use of multiple or external domains, they implicitly (and sometimes explicitly) advise managing the reputation of all linked entities. The documentation highlights that ISPs use sophisticated algorithms to evaluate trustworthiness based on comprehensive data, emphasizing that poor reputation from any included domain can lead to filtering.
Key findings
Domain reputation is central: Documentation often lists domain reputation as a primary factor influencing inbox placement, alongside IP reputation.
Holistic evaluation: Many providers indicate that sender reputation is assessed by considering various factors, including domain health, authentication, and content (which implies linked domains are also scrutinized).
Impact of third-party content: Documentation implies that if an email uses third-party content or links, the reputation of those third-party domains will factor into the email's overall score.
Subdomain benefits: Some documentation (e.g., AWS) suggests compartmentalizing sending via different domains or subdomains for various email types to optimize deliverability.
Blacklisting risks: Official guides warn that a low IP or domain reputation can lead to being blacklisted (or blocklisted), severely impacting email delivery.
Key considerations
Maintain clean domains: All domains used in an email, especially those handling sending, tracking, or content linking, should maintain a clean history and strong reputation. This aligns with advice on understanding your email domain reputation.
Implement authentication: Proper configuration of SPF, DKIM, and DMARC for all sending domains and subdomains is critical for authentication and building trust. Our guides provide detailed steps for safely transitioning DMARC policies.
Monitor blocklists: Regularly check if any of your sending or linked domains appear on common email blocklists (or blacklists). Being listed can cause significant deliverability issues (see Mailchimp's guide on sender reputation for more).
Segment sending patterns: For diverse email programs, using distinct subdomains for different types of emails can help isolate reputation. For instance, sending promotional emails from promo.yourdomain.com and transactional from trans.yourdomain.com helps mitigate risk if one segment encounters issues.
Technical article
Documentation from Amazon Web Services (AWS) advises that for optimal deliverability, it's beneficial to compartmentalize email sending across different domains or subdomains, especially for distinct email categories (e.g., marketing versus transactional).
22 Jun 2020 - AWS
Technical article
Customer.io documentation highlights that a low IP or domain reputation is a primary cause for emails being filtered to spam. They emphasize the importance of list health and engaged subscribers in maintaining a positive reputation.
