How does increasing volume on a shared IP affect my sender reputation?

Increasing volume too quickly on a shared IP can alert mailbox providers to unusual sending patterns, potentially triggering spam filters. Other senders on the same IP can also negatively impact your reputation, regardless of your actions. ISPs prefer consistent, gradual increases.

What is IP warming, and is it necessary for shared IPs?

IP warming is the process of gradually increasing email volume over time to build a positive sender reputation with ISPs. Yes, it's necessary even on shared IPs, as a sudden large jump in your specific sending volume can still be flagged as suspicious, leading to filtering or throttling of your emails.

Can a 3% bounce rate really cause a massive open rate drop?

While 3% might seem low, consistent hard bounces (invalid addresses) signal poor list hygiene to ISPs over time. This can accumulate into a negative sender reputation. When combined with a sudden increase in sending volume, it can exacerbate deliverability issues and lead to a significant drop in open rates, as ISPs may decide to filter more aggressively.

What data should I request from my ESP to troubleshoot this problem?

Request detailed bounce messages and open rates per recipient domain from your ESP. This data helps identify if the issue is global or isolated to specific mailbox providers. You should also check for any IP or domain blocklistings (blacklistings) and review your Google Postmaster Tools data.

Why did my email open rates drop after increasing sending volume on a shared IP? - Troubleshooting - Email deliverability - Knowledge base

It's a frustrating scenario many email senders face: your campaigns are performing well, you decide to scale up, and then suddenly, your open rates plummet. This is especially perplexing when you're using a shared IP and have been carefully warming your sending reputation. When I hear about a significant drop like going from 20-30% open rates down to 8% after increasing sending volume from 6,000 to 16,000 emails, even with a seemingly good domain reputation, it immediately flags a few common deliverability tripwires related to shared IPs and volume changes.

The transition from steady, lower volumes to a larger send can trigger alarms with Internet Service Providers (ISPs), even if your list quality seems consistent. Let's explore the key factors that contribute to this sudden decline and how you can diagnose and address them.

The challenges of shared IP addresses

One of the primary suspects when open rates drop after increasing volume on a shared IP is the nature of shared IP addresses themselves. While convenient for smaller senders, shared IPs mean your sending reputation is tied to the practices of other senders using the same IP. If another sender on your shared IP starts sending spam, hits spam traps, or experiences a sudden surge in complaints, it can negatively impact the reputation of the entire IP, affecting your deliverability even if your own practices are stellar. Mailbox providers like Google monitor the activity of all senders on a shared IP address, and a negative reputation can easily impact your inbox placement.

Even if your ESP promises a higher shared IP pool, the risk of collateral damage remains. An increase in your own sending volume, while part of a legitimate growth strategy, might also coincide with an adverse event from another sender on the same IP. ISPs might see the combined increased volume and a sudden spike in issues from one sender as a collective threat, leading to filtering for all users on that IP.

It's essential to understand that while Google might have upgraded your domain reputation to high, your IP reputation is a separate, albeit related, entity. A clean domain reputation doesn't always fully compensate for a struggling IP, especially on shared infrastructure. You might need to check your IP address against common blacklists (or blocklists) to see if it has been listed, even for minor reasons.

The impact of sudden volume increases

Even with an established sending history and a good domain reputation, a sudden jump in email volume can be problematic, especially on a shared IP. This is where IP warming principles come into play, even if you’re not on a brand-new, dedicated IP. ISPs look for consistent sending behavior. A sudden increase, like from 6,000 to 16,000 emails, can appear suspicious, even if the content and list quality remain the same. This abrupt change might cause ISPs to temporarily throttle or filter your emails to the spam folder, resulting in a severe drop in open rates.

Smart volume scaling on shared IPs

While shared IPs often handle warming internally, large jumps from your specific sending patterns can still cause issues. Even on a shared IP, it's wise to ramp up your volume gradually. This allows ISPs to adjust to your sending patterns and prevents your volume increase from being mistaken for a spam burst.

If you experience a drop, immediately revert to your last successful sending volume and slowly increase it again, monitoring your metrics closely. A good rule of thumb is to increase volume by no more than 10-20% per day or per send until you reach your desired level. For a detailed strategy, consider our guide on managing sending volume for larger email campaigns.

The introduction of a new sender name, even from the same domain with verified authentication, could also play a small role. Mailbox providers build reputation not just around the domain and IP, but also sender From addresses. A brand new From address, even under an established domain, may need its own micro-warming period if the volume is significant for that specific sender.

List quality and recipient engagement

A 2-3% bounce rate might seem acceptable, but consistent bounces, especially hard bounces (invalid addresses), can significantly impact your sender reputation over time. Mailbox providers (ISPs) see frequent bounces as a sign of poor list hygiene and potentially irresponsible sending practices. If they constantly detect a certain percentage of non-existent mailboxes in your traffic, they will assume your list quality is low and start filtering your messages. This can be a gradual erosion of trust that suddenly becomes critical when combined with other factors, like increased volume.

Engagement metrics are paramount to deliverability. Open rates, click-through rates, and even replies signal to ISPs that your emails are valued by recipients. If your list contains a significant number of unengaged recipients, even if they don't bounce or complain, it can pull down your overall engagement metrics. When you increase your sending volume, you are sending more emails to this potentially unengaged segment, which can dilute your positive signals and lead to lower inbox placement. This is often why we see a sudden drop in email open rates even with good overall metrics.

Prune your list: Regularly remove inactive or unengaged subscribers. Consider a re-engagement campaign before removing them entirely. Even a small percentage of invalid or unengaged addresses can disproportionately affect your sender reputation as volume increases.
Segment based on engagement: When increasing volume, prioritize sending to your most engaged segments first. This helps reinforce positive sending signals to ISPs.
Monitor bounce reasons: Exact bounce messages can provide critical clues. If you see a rise in 'spam content' or 'recipient policy reasons' bounces, it points to content or reputation issues.

Deeper diagnostics and ongoing monitoring

To effectively troubleshoot, you need granular data. Without the ability to filter open rates and bounce reasons by recipient domain, you're flying blind. Ask your ESP for detailed bounce codes, which can pinpoint if the issue is with a specific domain, content, or a broader reputation problem. For instance, a high bounce rate from

Outlook.com or

Gmail.com could indicate specific issues with those providers.

While you confirmed SPF and DKIM are set up for your domain, ensure DMARC is also correctly implemented and monitored. This suite of authentication protocols (SPF, DKIM, DMARC) is crucial for building and maintaining sender trust. If you haven't already, actively monitor your DMARC reports to catch any authentication failures or misconfigurations that might be contributing to your deliverability issues, especially after a migration or volume increase.

Additionally, consider that content can still play a role. Even if content hasn't changed, an increase in volume might expose it to more stringent spam filters if other reputation signals are weak. A/B test different subject lines and email body content to see if any specific elements are triggering filters. Faulty links in content can also be a hidden culprit. Finally, consider using a free email deliverability tester to get insights into how your emails are perceived by various mailbox providers.

Views from the trenches

Best practices

Always warm up your sending volume, even on shared IPs, to establish a consistent pattern.

Regularly clean your email lists to remove inactive or bouncing addresses and improve list quality.

Monitor your domain and IP reputation using tools like Google Postmaster Tools and blocklist checkers.

Prioritize sending to your most engaged segments when increasing volume to maintain positive signals.

Common pitfalls

Increasing sending volume too quickly can signal spam-like behavior to ISPs, regardless of content.

Neglecting list hygiene, leading to high bounce rates and poor sender reputation over time.

Underestimating the impact of other senders on a shared IP on your own deliverability.

Not analyzing bounce reasons and per-domain open rates for targeted troubleshooting.

Expert tips

If open rates drop, immediately scale back to your last successful sending volume and gradually ramp up.

Request granular bounce data and per-domain performance metrics from your ESP for deeper insights.

Continuously monitor all email authentication (SPF, DKIM, DMARC) for proper alignment.

A/B test subject lines and content to identify potential spam triggers, especially with new sender names.

Expert view

Expert from Email Geeks says to check IP blacklistings first, as shared IP pools can be affected by other senders, leading to a sudden drop in deliverability, even if your own volume is low. It's also important to determine if the open rate drop is isolated to a particular network, such as Verizon or Yahoo, or if it's spread across all domains.

2020-07-14 - Email Geeks

Marketer view

Marketer from Email Geeks mentioned that despite being in a supposedly higher shared IP pool, their IP check showed green except for timeouts with BLC, Madavi, and NoSolicitado, indicating some potential, albeit minor, blocklist issues. They also noted the difficulty in generating per-network data for open rates.

2020-07-14 - Email Geeks

Getting your emails back on track

A sudden drop in open rates after increasing sending volume on a shared IP is a common, but often multi-faceted, problem. It's rarely due to a single cause, but rather a combination of factors related to shared IP dynamics, the speed of your volume increase, and the underlying health of your email list and sending practices. Don't let it derail your email program. Instead, use it as an opportunity to review and strengthen your entire deliverability strategy.

By systematically investigating each potential cause, gathering the necessary data, and adjusting your sending approach, you can restore your open rates and ensure your emails consistently reach the inbox. Remember, patience and diligent monitoring are key to navigating the complexities of email deliverability.