How does sender domain vs IP hard bounce rate impact sender reputation with ISPs like Gmail?
Matthew Whittaker
Co-founder & CTO, Suped
Published 24 Jul 2025
Updated 16 Aug 2025
7 min read
When managing email deliverability, understanding how different factors impact your sender reputation is crucial. A key area of concern is hard bounce rates, which signal undeliverable emails due to permanent reasons. The question often arises, especially when operating with shared sending infrastructure: does the hard bounce rate at the sender domain level or the sender IP level carry more weight with Internet Service Providers (ISPs) like Gmail? This distinction is particularly important when attempting to re-engage older, potentially stale lists, where a sudden spike in bounces for a specific domain might occur while the overall IP's bounce rate remains stable.
The answer, as with many aspects of email deliverability, is nuanced and depends on the specific ISP and the nature of your sending operations. However, recent trends indicate a growing emphasis on domain reputation, particularly for major mailbox providers such as Gmail.
Understanding sender reputation: IP versus domain
Sender reputation is a critical metric that ISPs use to determine whether to deliver your emails to the inbox, spam folder, or reject them entirely. It is essentially a trust score, built over time based on your sending behavior. This score is typically composed of two main elements: IP reputation and domain reputation.
IP reputation is tied to the Internet Protocol (IP) address from which your emails originate. If a particular IP address is associated with spam, phishing, or high bounce rates, its reputation will suffer, impacting all senders using that IP. Domain reputation, conversely, is linked to your sending domain. This includes your 'From' address domain, the domain of links within your emails, and the domain used for email authentication records like SPF, DKIM, and DMARC. It reflects how recipients interact with emails from your brand, regardless of the specific IP used.
The distinction between these two types of reputation can be particularly significant when considering how various mailbox providers evaluate incoming mail. To learn more, delve into the difference between IP and domain reputation.
IP reputation
Foundation: Tied to the physical sending server's IP address.
Impact: Affects all domains sending from that IP. Crucial for new senders warming up IPs.
Volatility: Can fluctuate based on volume and quality of mail sent by all entities sharing the IP.
Domain reputation
Foundation: Linked to your email domain (e.g., yourcompany.com).
Impact: Follows your brand regardless of the underlying IP address.
Sustainability: More persistent and accumulates over the long term based on subscriber engagement and feedback.
The impact of hard bounces on reputation
Hard bounces are emails that cannot be delivered to a recipient's inbox due to a permanent reason. This typically means the email address is invalid, nonexistent, or has been permanently blocked. Unlike soft bounces (temporary issues), hard bounces immediately and severely impact your sender reputation. ISPs interpret high hard bounce rates as a strong indicator of poor list quality, outdated data, or even spamming practices. This can lead to your emails being flagged as spam, throttled, or even blocklisted (blacklisted).
A continuous high rate of hard bounces can severely damage both your IP and domain reputation. Mailbox providers, including Gmail, constantly monitor these metrics. They want to ensure their users receive legitimate emails and will block or filter senders who appear to be sending to invalid addresses. We explore this further in our guide on how hard bounces impact deliverability.
While both IP and domain hard bounce rates matter, the nuance lies in which one an ISP prioritizes. It's not always about the raw numbers across an IP, but how those bounces reflect on the specific sender's domain or brand. For instance, if an ISP sees a high number of hard bounces for a particular domain, it might infer that the sender is neglecting good sending practices or is using old lists, regardless of the IP's overall performance. Understanding the actual bounce reasons (the SMTP codes and messages) is crucial, as a 550 5.7.1 error due to a spam block is far worse than a 550 5.1.1 for 'user unknown'.
Hard bounce rate benchmarks
Aim to keep your hard bounce rate below 1% for optimal sender reputation. Rates between 2% and 5% are warning signs, while anything above 5% signals a serious issue requiring immediate action. Continuously sending to invalid addresses will lead to significant deliverability problems and potential blocklisting, regardless of the overall IP performance.
Gmail's perspective: domain-centric reputation
For Gmail, there's strong evidence that domain reputation is a primary factor in their filtering decisions. While IP reputation is still considered, the domain associated with your emails plays a significant role in how Gmail treats your mail. This means that a spike in hard bounces at the domain level can be more detrimental to your sending reputation with Gmail than a generalized, lower spike across a shared IP.
For example, if you're rehabilitating an old list, and the hard bounce rate for the specific sending domain jumps from 0.05% to 2%, even if the overall IP's hard bounce rate remains below 0.2%, Gmail will likely notice the domain-specific issue. While Gmail might tolerate a temporary spike if followed by consistent, clean sending, persistent high hard bounces for a domain could lead to increased spam filtering or even outright blocks for that domain. Our article on Gmail's prioritization of domain or IP reputation provides more detail.
Monitoring your GmailPostmaster Tools is essential for understanding your domain's health with Gmail. These tools provide detailed insights into your domain and IP reputation, spam rates, and bounce rates. Paying close attention to the specific bounce codes and text messages from Gmail can help you diagnose issues accurately. For example, a 550-5.7.1 bounce indicating likely unsolicited mail requires different action than a 550 5.1.1 for a nonexistent user.
Understanding SMTP bounce codes
SMTP bounce codes and their accompanying text messages provide crucial information about why an email failed to deliver. A generic 550 5.7.1 status, for example, could mean either a missing From: header or that the message is considered unsolicited. The specific text message dictates the necessary fix, highlighting the importance of detailed reporting beyond just the numerical code. The SMTP Field Manual is a valuable resource for deciphering these.
Example Gmail SMTP Bounce Message
550-5.7.1 [redacted_ipv4] Our system has detected that this message is 550-5.7.1 likely unsolicited mail. To reduce the amount of spam sent to Gmail, 550-5.7.1 this message has been blocked. Please visit https://support.google.com/mail/?p=UnsolicitedMessageError 550 5.7.1 for more information.
Managing hard bounces with shared IPs
When using shared IP pools, it's common to see your domain's hard bounce rate spike even if the overall IP's bounce rate remains low. This is because the shared IP distributes the load and reputation impact across many domains. However, ISPs (like Gmail) are sophisticated enough to track reputation at both levels. A consistently high hard bounce rate for your specific domain, even on a well-reputed shared IP, will still degrade your domain's standing.
To mitigate risks when activating old lists or encountering sudden bounce spikes, focus on list hygiene and segment your sends. Instead of sending to the entire stale list at once, gradually work through it. This allows you to monitor bounce rates and other engagement metrics in real-time. If you observe a persistent hard bounce rate above 1-2% for a particular segment, it is critical to immediately cease sending to those addresses and review your list acquisition methods.
Email verification services can also significantly reduce hard bounces by identifying invalid or risky email addresses before you send to them. Regularly cleaning your list, removing inactive subscribers, and suppressing hard bounces are fundamental practices for maintaining a healthy sender reputation and avoiding emails going to spam.
Strategy
Description
Impact on Hard Bounces
List hygiene
Regularly remove inactive subscribers and addresses that have previously hard bounced. Use email verification services to pre-clean lists.
Directly reduces hard bounces by ensuring you send to valid addresses, improving overall sender reputation.
Segmented sending
Break large or old lists into smaller segments and send gradually, monitoring metrics after each send.
Limits the impact of a high bounce rate to a smaller portion of your sends, allowing for quicker adjustments and minimizing reputation damage.
Monitor delivery codes
Go beyond basic bounce flags. Analyze the specific SMTP bounce codes and accompanying text messages provided by ISPs.
Helps diagnose the root cause of bounces, differentiating between a 'user unknown' and a 'spam block', leading to targeted solutions.
Views from the trenches
Best practices
Always validate your email lists regularly, especially before sending to older segments, to preemptively identify and remove invalid addresses.
Monitor your sender reputation metrics, particularly hard bounce rates, through dedicated tools and ISP-specific feedback loops.
Segment your email campaigns and start with highly engaged recipients to build positive sending momentum and gradually expand.
Ensure that your email authentication records (SPF, DKIM, DMARC) are correctly configured to establish legitimate sending identity.
Common pitfalls
Sending to purchased or scraped email lists, which inevitably contain high numbers of invalid addresses and spam traps.
Ignoring hard bounce notifications, which signals to ISPs that you are not maintaining a clean and engaged subscriber list.
Failing to segment lists, leading to large sends to unengaged or stale recipients that can trigger high hard bounce rates.
Not analyzing detailed bounce codes and messages, which prevents you from understanding the precise reasons for delivery failures.
Expert tips
Implement a double opt-in process for all new subscribers to ensure high quality and genuine interest from recipients.
Utilize
logo.dev
Google Postmaster Tools
Expert view
Expert from Email Geeks says the impact of hard bounce rates varies depending on the specific mailbox provider.
2021-10-14 - Email Geeks
Expert view
Expert from Email Geeks says the severity of the impact also correlates directly with the bounce rate percentage itself.
2021-10-14 - Email Geeks
Prioritizing reputation: domain over IP for hard bounces
While both IP and domain hard bounce rates contribute to your overall sender reputation, Gmail places a significant emphasis on your domain's reputation. This means a spike in hard bounces at the domain level, even if the overall IP's bounce rate remains low, can still negatively impact your deliverability with key mailbox providers. Proactive list management, regular hygiene, and meticulous monitoring of bounce reasons are essential to maintaining a healthy sending reputation and ensuring your emails reach the inbox.
Gmail? This distinction is particularly important when attempting to re-engage older, potentially stale lists, where a sudden spike in bounces for a specific domain might occur while the overall IP's bounce rate remains stable.
email verification services to pre-clean lists.
