What is a shared IP address and how does it affect email deliverability?

While shared IPs can be cost-effective and simpler to manage for lower volume senders, they introduce complexities regarding email deliverability. The reputation of a shared IP is influenced by the sending practices of all users on that IP, meaning one bad sender could potentially affect the inbox placement of others. Understanding this dynamic is crucial for maintaining good sender reputation and ensuring your emails reach their intended recipients.

Key findings

• Collective reputation: The deliverability of emails sent from a shared IP address is directly tied to the collective sending reputation of all users on that IP. If one sender engages in problematic behavior, it can negatively impact everyone else sharing the same IP, potentially leading to increased spam classifications or blocklistings.
• ESPs manage reputation: Email service providers typically manage shared IP pools. They are responsible for monitoring and maintaining the reputation of these IPs by grouping senders with similar sending practices and volume, and by removing malicious or problematic users. For more on how ESPs handle IPs, see our guide on how ESPs impact deliverability on dedicated IPs.
• Reduced control: Senders using shared IPs have less direct control over their IP reputation compared to those using dedicated IPs. Their deliverability is partly dependent on the behavior of unknown co-senders.
• Historical context: In earlier periods of email, IP reputation was a more dominant factor in deliverability. However, modern inbox service providers (ISPs) increasingly rely on domain reputation and engagement metrics, which somewhat mitigates the risks associated with shared IPs, as detailed by articles like this one on SendGrid's blog.

Key considerations

• Sending volume and consistency: Shared IPs are often recommended for senders with lower, less consistent volumes, as the pooled traffic helps smooth out fluctuations. High-volume or highly consistent senders might benefit from dedicated IPs.
• List hygiene: Regardless of IP type, maintaining a clean and engaged email list is paramount. Poor list hygiene on a shared IP can quickly degrade the shared reputation for all users, leading to blocklistings (or blacklistings). Learn more about what happens when your IP gets blocklisted.
• Monitoring and reporting: While you can't control other senders, you can actively monitor your own email performance metrics, such as open rates, click-through rates, and complaint rates. This helps in identifying and addressing any issues with your sending practices before they significantly impact the shared IP's reputation.
• ESP reputation management: A reputable ESP will have robust mechanisms to protect their shared IP pools, including strict acceptable use policies and proactive monitoring for spam and abuse. Choosing a reliable ESP is critical for senders on shared IPs.

However, experienced marketers recognize the inherent risks. The lack of direct control over the IP's reputation due to the actions of other senders is a significant concern. This leads to a strong emphasis on maintaining impeccable sending hygiene and careful monitoring of campaign performance, even when relying on an ESP's shared infrastructure. The consensus leans towards shared IPs being suitable under specific conditions, but with an ongoing awareness of potential deliverability fluctuations tied to the collective sending pool.

Key opinions

• Ease of entry: Marketers appreciate shared IPs for simplifying the technical aspects of email sending, allowing them to focus more on content and strategy rather than IP reputation management, particularly for smaller campaigns. This can be beneficial when you are deciding between shared and dedicated IPs.
• Volume considerations: For low-volume senders, shared IPs can provide a stable sending environment, as their individual volume might not be enough to establish a strong dedicated IP reputation quickly.
• Reputation vulnerability: A primary concern is the risk of deliverability issues caused by other, potentially abusive, senders on the same IP pool. This external influence can lead to unexpected inboxing problems or even being put on a blocklist.
• ESP role: Marketers heavily rely on their ESPs to actively monitor and manage the shared IP reputation, ensuring problematic senders are quickly identified and mitigated to protect the overall pool. Learn more about Spamcop reports on shared IPs.

Key considerations

• Vigilant monitoring: Despite ESP management, marketers on shared IPs should diligently track their own email performance, including open rates, clicks, complaints, and bounces, to catch any signs of deliverability issues early.
• Audience segmentation: Using shared IPs for highly engaged segments can help build positive domain reputation, which can partially offset IP-related risks. Conversely, sending high-spam emails from a shared IP can negatively impact other campaigns, emphasizing the need to isolate problematic sending.
• Domain reputation importance: Focusing on building and maintaining a strong domain reputation becomes even more critical on shared IPs, as it serves as a more stable indicator of sender trustworthiness to ISPs.
• Compliance: Strict adherence to anti-spam laws and best practices, such as obtaining explicit consent and providing easy unsubscribe options, is vital to protect both your own sender reputation and the shared IP's health.

For a deeper dive, consider this article on 5 questions and answers about shared IP addresses.

Today, inbox service providers (ISPs) increasingly prioritize domain reputation and engagement metrics over raw IP reputation, particularly for consumer mailbox providers. This doesn't mean IP reputation is irrelevant; rather, it indicates a more nuanced approach where the actions of individual senders on a shared IP are often filtered or mitigated by the overall domain health and user interaction. Experts emphasize that while shared IPs are less of a deal-breaker than they once were, proactive reputation management by the ESP remains crucial.

Key opinions

• Evolving IP importance: Experts acknowledge that IP reputation, especially on shared IPs, has become less singularly crucial since around 2015 compared to older times (pre-2010), particularly with consumer mailbox providers. Domain reputation and engagement now play a more significant role in filtering decisions.
• Shared pools are common: Most ESPs operate a mix of shared and dedicated IP pools, reflecting the varied needs of their customer base. Shared pools are a standard offering for many sending scenarios.
• Risk of collateral damage: While less severe than in the past, there's still a risk that a problematic sender on a shared IP could negatively affect others' deliverability. Historically, manual IP address blocklisting was a significant concern that could halt email flow entirely.
• Domain reputation as a buffer: The increasing reliance on domain reputation means that even on a shared IP, a strong domain reputation can help mitigate the impact of a less-than-perfect IP. This emphasizes the importance of consistent good sending practices from your domain, a concept explored further in articles like how domain and IP reputations interact.

Key considerations

• Choose a reputable ESP: The quality of the ESP's shared IP management is paramount. A good ESP will actively monitor its shared pools, quickly identify and address senders engaging in abusive practices, and filter out undesirable traffic. This active management protects legitimate senders on the shared IP.
• Segment sending: Even on shared IPs, segmenting your audience and sending targeted, engaging content can significantly improve your individual sender metrics, which in turn benefits your domain reputation.
• Understand the balance: While IP reputation is not the sole factor, it still contributes to deliverability. Senders should understand the interplay between IP, domain, and content factors. Our guide on how SPF, DKIM, DMARC, and dedicated IPs affect deliverability delves into this technical balance.
• Monitor your own metrics: Even with an ESP managing shared IPs, individual senders must continuously monitor their own sender performance and inbox placement. This proactive approach allows for quick adjustments to sending strategy if issues arise.

For more on the evolution of IP importance, an article by Mailgun discusses dedicated vs. shared IPs.

Documentation often clarifies that while shared IPs offer benefits like immediate reputation leverage and reduced warming periods, they also come with the caveat of shared reputation. The ESP is positioned as the guardian of this shared reputation, implementing measures to mitigate risks from abusive senders and ensure the overall health of the IP pool. Despite this management, senders are advised to maintain strong sending practices and monitor their own performance metrics.

Key findings

• Definition: Shared IP pools are defined as groups of IP addresses collectively used by multiple email senders, allowing them to send marketing campaigns or other email types without needing individual IP setup.
• Suitability: They are often described as ideal for companies with occasional or smaller-volume mailing needs, as the aggregated traffic from many senders can help establish a stable sending volume that benefits the IP's reputation from the outset.
• Reputation exposure: Documentation consistently warns that senders on a shared IP do not have full control over their sender reputation and email deliverability. Instead, their standing is partially determined by the collective behavior of other users in the pool.
• ISP bundling: Shared IP addresses are typically bundled together by ESPs and used to send mail equally across the group. This helps distribute traffic and manage the overall load and reputation.

Key considerations

• Trust in ESP: Senders must place significant trust in their chosen ESP to diligently manage the shared IP pool. This includes rigorous onboarding, continuous monitoring, and quick action against any abusive senders who might jeopardize the shared reputation.
• Impact of bad actors: If a user on a shared IP engages in spammy practices, the entire IP address (or the sub-pool) can become blocklisted, affecting the deliverability of all legitimate senders, underscoring the communal risk.
• Domain reputation's role: While IP reputation is key, ISPs increasingly weigh domain reputation and recipient engagement. Therefore, documentation advises focusing on these elements to bolster overall deliverability, even on shared IPs. This is also why an article from Klaviyo mentions recipient engagement as a factor.
• Ongoing monitoring: Even with shared IPs, continuous monitoring of email campaign performance, including bounces, complaints, and engagement rates, is essential for individual senders to ensure their practices align with deliverability best practices.