How do shared IP pools and sending domains impact email sender reputation for ESPs?
Michael Ko
Co-founder & CEO, Suped
Published 13 Jul 2025
Updated 16 Aug 2025
8 min read
Understanding how shared IP pools and sending domains interact is fundamental for Email Service Providers (ESPs). These elements are not just technical configurations, but critical determinants of an ESP's overall sender reputation and, consequently, its ability to deliver emails reliably to the inbox.
The complex interplay between IP reputation and domain reputation means that ESPs must constantly balance the collective sending practices of their diverse client base. Mailbox providers (MBPs) assess these various elements to decide whether an email is legitimate or spam, directly impacting inbox placement for all senders on the platform.
The dual nature of email reputation
IP reputation is intricately tied to the numerical address from which emails originate. It serves as an initial filtering mechanism for MBPs, enabling them to quickly identify and manage known sources of problematic traffic. Domain reputation, conversely, is linked to your sending domain, the one displayed in the 'From' field of an email. This reputation metric typically exhibits more persistence and offers a clearer reflection of long-term sending practices and audience engagement patterns.
When an ESP manages email sending, several crucial domains come into play. The 5322.From domain is what recipients see. The 5321.MAILFROM, or Return-Path, domain is primarily used for bounce handling and is usually under the ESP's control. Additionally, DKIM signing domains carry significant reputational weight. These elements collectively form a comprehensive picture of a sender's trustworthiness. Our article on RFC 5322 provides more detail.
Beyond the visible sender domains, any domains utilized for tracking links (click and open tracking) or for hosting images within emails also contribute to the overall reputation assessment. If these auxiliary domains are controlled by the ESP, their reputation can indirectly affect client deliverability. This intricate web of identifiers means a holistic approach to email security is essential.
Multiple factors influence reputation
Email sender reputation is not a single score but a complex evaluation by mailbox providers. They analyze various identifiers associated with an email to determine its trustworthiness.
IP address: The numerical address of the sending server.
5322.From domain: The visible sender domain.
5321.MAILFROM domain: The bounce address (Return-Path), often an ESP's domain.
DKIM signing domain: The domain that digitally signs the email.
Tracking domains: Domains used for click and open tracking.
Shared IP pools and their complexities
Shared IP pools are groups of IP addresses used by multiple senders simultaneously. ESPs often utilize them to distribute sending volume, warm up new IPs, and simplify deliverability management for clients, especially those with lower sending volumes or who are new to email marketing. This setup can be a cost-effective solution and provides inherent warming from other senders' traffic. You can learn more about what a shared IP address is and its impact.
The main challenge with shared IP pools is the "bad neighbor" effect. If one sender in the pool engages in poor sending practices, such as sending to spam traps, generating high complaint rates, or having significant bounce rates, it can negatively impact the reputation of the entire shared IP. Mailbox providers like Gmail often apply the reputation of the worst sender to the entire IP. This is why ESPs must work diligently to maintain clean shared IP pools. You can read more about how your IP reputation is influenced by others on a shared IP address.
To mitigate this risk, many ESPs employ sophisticated customer segregation strategies. They might place "good" customers, who consistently maintain strong engagement and low complaint rates, on one set of shared IPs, while "risky" or new customers might start on another. If a customer's sending practices deteriorate, they may be moved to a different, lower-reputation pool or, in severe cases, off the platform entirely. This allows MBPs to treat mail more granularly based on the specific IP pool's performance.
Shared IP pools
With shared IP pools, your email deliverability is influenced by the collective sending behavior of all users on that IP.
Reputation risk: Susceptible to other senders' poor practices.
Warming: Can leverage existing IP reputation (pre-warmed).
Control: Less control over IP reputation management.
Cost: Generally more affordable for ESP users.
Dedicated IPs
A dedicated IP offers exclusive control over your sending reputation. Your practices directly dictate its health.
Reputation control: Full control, but also full responsibility.
Warming: Requires a careful, gradual IP warming process.
Impact: Poor sending practices immediately harm your reputation.
Suitability: Best for high-volume, consistent senders.
Sending domains and ESP reputation
While IP reputation is crucial, domain reputation has become increasingly important. It acts as a stabilizing force, often providing MBPs with a more precise indicator of a specific sender's long-term trustworthiness, regardless of the underlying IP address. This is why a strong domain reputation can sometimes help a sender "survive" on a shared IP with a less-than-stellar collective reputation. Learn about how domain reputation affects deliverability compared to IP reputation.
A common question arises regarding the ESP's own domain, especially if customers use their unique sending domains. Even if a customer sends emails from their own branded domain (e.g., yourcompany.com), the ESP may still use its own domain for the 5321.MAILFROM address, or for hosting tracking links and images. If multiple clients on an ESP's network engage in harmful sending practices, it can potentially damage the ESP's root domain reputation, affecting all its clients.
Fortunately, for most reputable ESPs, their default 5321.MAILFROM and shared DKIM domains typically do not significantly factor into an individual sender's reputation, unless the ESP itself is known for being "spammer friendly." ESPs use these domains primarily for system functions like bounce processing and feedback loops. However, if an ESP doesn't rigorously police its customers, its shared domain could eventually face reputation problems. Our guide on how an ESP's shared domain can affect deliverability provides more context.
Factor
Description
Primary Impact
IP address
The numerical address of the sending server.
Determines immediate trust and can lead to quick blocklisting if poor. Shared IPs spread risk.
Sending domain (5322.From)
The domain visible as the sender in the email client.
Long-term trust, brand recognition, and a primary factor for inbox placement. More stable reputation.
MAILFROM domain (5321)
The return-path or bounce address.
Often ESP-controlled for bounce handling. Less direct impact on individual sender reputation, unless ESP is problematic.
DKIM signing domain
The domain used for digital signature authentication.
Crucial for authentication and validating email origin. Strong indicator of legitimate sending.
Tracking domains
Domains for click and open tracking links.
Can reflect overall email quality if compromised or associated with malicious activity. Best when branded.
Mitigating risks and maintaining reputation
For ESPs, proactively managing shared IP pools is paramount. This involves careful client onboarding, robust monitoring of sending practices, and dynamic traffic segmentation. By promptly identifying and isolating abusive senders, ESPs can protect the integrity of their shared infrastructure and maintain high deliverability rates for their compliant clients. Effective management of IP pools is vital for deliverability.
For senders using ESPs, maintaining excellent sending hygiene is critical, regardless of whether you're on a shared or dedicated IP. This means ensuring high subscriber engagement, minimizing spam complaints, promptly removing invalid addresses, and adhering to permission-based sending practices. Your email sending practices directly impact your domain reputation.
Another key aspect is email warming. Whether you are using a shared IP (especially a new pool) or a dedicated IP, gradually increasing your sending volume to new mailbox providers helps establish a positive reputation. This process builds trust and shows MBPs that your sending is legitimate and consistent, preventing sudden blocks or throttling. Microsoft provides an excellent guide on email warm-up processes.
Regularly monitoring your sender reputation across various blocklists (or blacklists) and leveraging DMARC reporting are indispensable. Blocklist monitoring helps identify if your IPs or domains have been flagged, allowing for swift action. DMARC provides crucial insights into authentication failures and potential abuse, empowering you to protect your domain's reputation. These are crucial steps for any serious sender.
Implement robust client onboarding and monitoring processes.
Segment clients into appropriate shared IP pools based on sending behavior.
Educate clients on maintaining high subscriber engagement and low complaint rates.
Use dedicated domains for tracking links to avoid shared reputation issues.
Regularly monitor IP and domain reputations for any signs of degradation.
Common pitfalls
Failing to adequately police client sending practices on shared IPs.
Not segregating problematic senders from good ones within shared pools.
Ignoring negative reputation signals on ESP's own domains.
Allowing unbranded third-party domains in message bodies or tracking links.
Neglecting to warm up new IPs or domains properly.
Expert tips
Sender reputation relies on multiple factors including DKIM, IPs, and the 5322.From domain.
If an ESP fails to segregate customers by domain after segregating by IP, long-term domain problems can arise.
ESPs should ensure their own domain is used for DKIM signing and bounce handling for clear identification.
Tracking domains should ideally be delegated to the sender's own domain to minimize third-party reputation exposure.
Proactive management and traffic segmentation are key for ESPs to protect shared IP reputation.
Expert view
Expert from Email Geeks says: Sending reputation is calculated using many data points, commonly built on the DKIM signing domain, the sending IPs, and the 5322.From domain, not the 5321.MAILFROM domain.
2023-05-03 - Email Geeks
Expert view
Expert from Email Geeks says: If an ESP segregates by IP but never by domain, they will eventually face domain problems if customer policing isn't perfect, as observed firsthand at Gmail.
2023-05-03 - Email Geeks
Maintaining email sender reputation
The interplay between shared IP pools and sending domains is a cornerstone of email deliverability for ESPs. While IP reputation provides an immediate signal of trust or risk, domain reputation offers a more enduring and stable measure of sender trustworthiness. ESPs play a vital role in managing the collective health of their sending infrastructure, influencing both IP and domain health across all their clients.
Ultimately, successful email delivery hinges on a collaborative effort between ESPs and their clients, focused on consistent good sending practices, proper email authentication (SPF, DKIM, DMARC), and continuous monitoring of reputation metrics. This proactive and diligent approach ensures that emails reach the inbox and maintain a strong reputation in the long run.
Gmail often apply the reputation of the worst sender to the entire IP. This is why ESPs must work diligently to maintain clean shared IP pools. You can read more about how your IP reputation is influenced by others on a shared IP address.
