When to consider multiple dedicated IPs for large email lists and what causes deliverability degradation?
Matthew Whittaker
Co-founder & CTO, Suped
Published 11 Aug 2025
Updated 19 Aug 2025
9 min read
Managing email deliverability for large lists presents a unique set of challenges. As your subscriber base grows and sending volumes increase, relying on a single dedicated IP address can begin to limit your reach and even degrade your inbox placement. While a dedicated IP offers control over your sending reputation, there's a point where a single IP can no longer handle the throughput required by high-volume senders, potentially leading to delays and missed opportunities.
Many email senders experience a gradual decline in open rates or an increase in delayed deliveries as their mailing lists expand. Even with stringent email validation and engagement practices, hitting specific volume thresholds on a single IP can trigger flags with internet service providers (ISPs). Understanding when to scale your IP infrastructure and what factors contribute to deliverability degradation is crucial for maintaining a healthy email program.
Considering multiple dedicated IPs
A dedicated IP address means your sending reputation is entirely your own responsibility. For businesses sending consistent, high volumes of email, this control is invaluable. It allows you to build a stable and predictable sending history with ISPs. However, there's a physical limit to how much mail a single IP can send effectively within a given timeframe without encountering throttling or deferrals from recipient mail servers. When you exceed these implicit limits, ISPs may interpret the high volume from one IP as potentially suspicious, regardless of your list quality.
The primary reason to consider multiple dedicated IPs is to increase your throughput. Inbox providers often limit the number of emails they accept from a single IP per connection or per hour. By having multiple IPs, you effectively open more 'pipelines' to these providers, allowing you to send a larger volume of emails simultaneously. This is particularly critical for time-sensitive campaigns or when you need to deliver millions of emails within a very short window.
Beyond raw volume, multiple IPs offer advanced segmentation opportunities. For instance, you might use one IP (or a set of IPs) for your transactional emails, which typically have higher engagement rates and are crucial for business operations, and another set for marketing or promotional emails. This separation ensures that any deliverability issues with your marketing campaigns do not negatively impact the critical deliverability of your transactional messages. This strategic separation helps in maintaining dedicated IP reputation across different email streams.
When to consider multiple dedicated IPs
The decision to move to multiple dedicated IPs often comes when a single IP reaches its saturation point. While there isn't one universal number, general guidelines suggest that if you're consistently sending over 400,000 active subscribers per month, or around 100,000-200,000 emails per day, you should start evaluating the need for additional IPs. Signs that you might need more IPs include:
Delivery delays: Your emails are taking longer than usual to reach the inbox, indicating throttling by ISPs.
Decreasing open rates: Especially for major mailbox providers like Gmail, Hotmail, and Yahoo, which could signal inbox placement issues.
Growing subscriber base: Your sending volume is consistently increasing, pushing the limits of your current IP.
Diverse email types: Sending both promotional and transactional emails from the same IP can dilute its reputation.
It's not just about hitting a magic number, but also about the impact on your campaign performance. If a 6% drop in open rate translates to tens of thousands of missed opens, the investment in additional IPs and the IP warming process might be worthwhile. Consider if your email service provider (ESP) can help with this transition, as ESPs play a significant role in managing dedicated IPs and their reputation.
Single dedicated IP
Volume limitations: Risk of throttling or queueing for large sends.
Reputation risk: All email types (transactional, marketing) share the same reputation.
Engagement impact: Low engagement from one segment affects the entire sending stream.
Monitoring complexity: Harder to pinpoint specific campaign issues.
Multiple dedicated IPs
Increased throughput: Faster delivery for very high volumes.
Reputation segmentation: Separate IPs for different email types (e.g., marketing vs. transactional).
Improved stability: Mitigates the risk of one campaign affecting others.
Granular insights: Easier to diagnose and isolate deliverability issues.
Ultimately, the need for more IPs depends on your specific sending patterns, target audience, and desired delivery speed. Organizations needing to send millions of emails in a short period often find multiple IPs indispensable.
What causes deliverability degradation?
Even with optimal IP infrastructure, deliverability degradation can occur due to various factors. One of the primary culprits is a decline in sender reputation. This can be caused by:
Low engagement: If recipients aren't opening, clicking, or replying to your emails, ISPs see this as a sign of irrelevant content, leading to lower inbox placement. This is particularly true for large providers like Gmail, which prioritize user engagement signals.
High spam complaints: A surge in users marking your emails as spam is a direct signal of poor sender reputation. Even a small percentage can significantly harm your deliverability.
Spam traps: Hitting spam traps indicates a stale or poorly managed list. This can result in your IP or domain being added to a blacklist (or blocklist).
Content issues: Spammy keywords, broken links, or suspicious formatting can trigger filters.
Another often-overlooked factor is audience seasonality or changes in recipient behavior. For example, if you send to .edu email addresses, you might observe a natural drop in engagement during summer months when students are not actively checking their university emails. This seasonal fluctuation, while not a direct deliverability issue, can impact your engagement metrics and thus your perceived sender reputation.
Finally, ensure your email authentication protocols—SPF, DKIM, and DMARC—are correctly configured. Errors in these settings can lead to emails being rejected or sent to spam, even if your IP reputation is good. Monitoring your Google Postmaster Tools and DMARC reports can provide valuable insights into deliverability issues.
Strategies to combat deliverability degradation
When facing deliverability degradation, particularly with large lists, a multi-faceted approach is essential. Here are key strategies:
List segmentation and hygiene: Segment your list based on engagement. Regularly remove inactive subscribers and unengaged contacts. Implement strict email validation processes.
Monitor key metrics: Keep a close eye on open rates, click-through rates, bounce rates, and spam complaint rates. Pay particular attention to trends for specific domains like Gmail, Yahoo, and Outlook.
IP warming (if adding IPs): If you acquire new dedicated IPs, gradually increase your sending volume from them to build a positive reputation. This process, known as IP warming, is essential for new IP addresses.
Content optimization: Ensure your email content is relevant, personalized, and avoids common spam triggers. A/B test subject lines and content to improve engagement.
Authentication: Verify your SPF, DKIM, and DMARC records are correctly set up and aligned. These protocols are fundamental to email security and deliverability.
If you're already doing all these things and still seeing degradation, it's worth reviewing your sending volume against ISP limits and considering if additional IPs are the next logical step for your email list size. Always review bounce messages to understand the specific reasons for rejections, as they often provide direct clues, such as being listed on a blacklist (or blocklist like Proofpoint).
Example: Checking bounce messages
Bounce messages can be highly informative. Look for error codes and descriptions that point to the cause of delivery failure. For example, a 554 error often indicates a block. You might see messages similar to the following:
Example Bounce Message
smtp;554 Blocked - see https://ipcheck.proofpoint.com/?ip=###.###.###.###
Such messages explicitly tell you your IP is blocked and provide a link to check the specific blocklist, allowing for targeted delisting requests if appropriate.
Understanding the risks of volume peaks and dips on a dedicated IP is also key. Consistent sending volume helps maintain a stable reputation. Large, sudden increases or decreases in volume can be seen as anomalies by ISPs, impacting deliverability.
Final thoughts on scaling and deliverability
For email senders with large lists, navigating the complexities of IP strategy and deliverability degradation requires ongoing attention. While a single dedicated IP is suitable for many, reaching certain volume thresholds often necessitates expanding to multiple IPs to ensure consistent throughput and maintain a strong sender reputation. It is not just about the volume, but also the quality of your list, engagement metrics, and adherence to authentication standards that collectively determine your inbox placement.
Regularly monitoring your performance and understanding the specific causes of any deliverability drops, whether related to IP capacity, list hygiene, or seasonal factors, is key. By proactively managing your sending infrastructure and list quality, you can continue to achieve high inbox placement rates even with the largest email lists.
Views from the trenches
Best practices
Actively monitor open rates and delivery delays, especially for key mailbox providers like Gmail.
Segment your email sends across multiple dedicated IPs to separate transactional from marketing emails.
Regularly clean your email lists to remove unengaged subscribers and prevent hitting spam traps.
Common pitfalls
Underestimating the volume thresholds where a single IP becomes a bottleneck, causing delivery delays.
Ignoring subtle degradation in open rates, which can be an early sign of deliverability issues.
Failing to adapt sending strategies for seasonal changes, especially for lists with significant student populations.
Expert tips
If you observe delivery delays, it's a strong indicator that you might need additional IP addresses to handle throughput.
Gmail's reputation assessment often focuses more on content and engagement signals rather than just IP history.
Always check bounce messages for specific reasons, such as blocklist mentions, to diagnose issues effectively.
Expert view
Expert from Email Geeks says the need for multiple IPs depends on the required throughput. For instance, a campaign needing to send 12 million emails in 90 minutes would require multiple IPs because inbox providers limit the number of emails received from a single IP per connection.
2018-08-06 - Email Geeks
Expert view
Expert from Email Geeks says if you are not seeing delays in your mailing deployments, additional IPs may not be a strict necessity, but they can be beneficial for further reputation segmentation, even if current practices are excellent.
