How do soft bounces and unsubscribe rates impact Gmail sender reputation during IP warming?
Michael Ko
Co-founder & CEO, Suped
Published 15 Jun 2025
Updated 17 Aug 2025
7 min read
When you're warming up a new IP address (or even a new domain), every signal you send to Internet Service Providers (ISPs) like Gmail is magnified. It's like building a reputation from scratch, where every interaction counts. Two crucial metrics that often cause concern are soft bounces and unsubscribe rates. Understanding how these impact your sender reputation, especially with Gmail, is key to a successful IP warming strategy.
During IP warming, ISPs are closely watching your sending patterns and recipient engagement. They want to see that you're a legitimate sender, sending wanted mail to an engaged audience. Any signs of trouble, even temporary ones, can derail your warming efforts and lead to poor deliverability, meaning your emails might end up in the spam folder instead of the inbox.
Understanding soft bounces and their impact
Soft bounces occur when there's a temporary issue preventing an email from being delivered. Common reasons include a recipient's inbox being full, the server being temporarily down, or the message being too large. Unlike hard bounces (which are permanent failures due to invalid addresses), soft bounces suggest the email address is valid but currently unreachable. While soft bounces are temporary, their impact during IP warming can be significant.
While a single soft bounce won't sink your reputation, a high volume of them, especially for reasons like mailbox full, can signal underlying list quality issues to ISPs. Gmail looks at overall engagement. If you're consistently sending to mailboxes that are full, it suggests you're targeting inactive or neglected accounts. This can indirectly affect your sender reputation by indicating poor list hygiene or an unengaged audience, which ISPs dislike.
During IP warming, your goal is to send to your most engaged subscribers to build positive signals. If a significant portion of your soft bounces are due to full mailboxes, it implies that the segments you're using for warming might not be as engaged as you think, or the data is outdated. This undermines the purpose of warming and can lead to a negative reputation from the start. You can learn more about how soft bounce retry policies affect your sender reputation.
The role of unsubscribe rates in reputation
Unsubscribe rates are another critical metric during IP warming. While it might seem counterintuitive, an unsubscribe (when a user actively clicks an unsubscribe link) is generally viewed more favorably by Mailbox Providers (MBPs) like Gmail than a spam complaint.
A user clicking an unsubscribe link indicates that they no longer wish to receive your emails, but they recognize you as a legitimate sender. This active choice allows you to remove them from your list, preventing future negative interactions. On the other hand, if a user can't easily find or use an unsubscribe option, or if they're frustrated, they're more likely to mark your email as spam. Spam complaints are a strong negative signal that can severely damage your sender reputation and lead to emails landing in the spam folder.
During IP warming, a sudden spike in unsubscribe rates could indicate that your targeting is off or that your content isn't as relevant to the audience as you thought. While it's better than spam complaints, a high unsubscribe rate still points to user dissatisfaction. Gmail encourages senders to provide an easy way to unsubscribe, as it ultimately improves engagement for those who remain.
Impact during IP warming
The critical period of IP warming is when you're establishing trust with ISPs. Both soft bounces and unsubscribe rates, when poorly managed, can send negative signals. A common pitfall is using outdated or unengaged lists for warming, especially if there's been a long gap in sending.
For instance, if you're starting IP warming with a list that hasn't received emails in months, you're likely to encounter more full inboxes and higher unsubscribe rates. Recipients might not remember opting in, leading to disengagement and potentially spam complaints. This can quickly damage your newly forming IP (or domain) reputation.
The key is to use the cleanest, most recently engaged data available for your warming efforts. If your bounce rates are high or you're seeing unexpected unsubscribes during this phase, it's a clear signal to re-evaluate your list segmentation and sending strategy. This is especially true for Gmail, which heavily relies on engagement metrics to determine inbox placement.
Strategies for a strong sender reputation
Effective management of soft bounces and unsubscribe rates is paramount for a successful IP warming. Here are some strategies:
List hygiene: Use only your most engaged subscribers for warming, especially those who have opened or clicked an email recently (e.g., in the last 30-60 days). Regularly clean your lists to remove inactive users and hard bounces. This proactive approach ensures you're sending to recipients who actually want your emails, minimizing potential issues.
Unsubscribe visibility: Make your unsubscribe link clear and easy to find in every email. This reduces the likelihood of spam complaints, which are far more damaging than unsubscribes. Consider using a one-click unsubscribe header (List-Unsubscribe), which Gmail actively promotes.
Monitoring and adjustment: Closely monitor your deliverability metrics via Google Postmaster Tools for Gmail. If you see an increase in soft bounces or unsubscribes, consider reducing your sending volume or further segmenting your audience. This agility is vital for successful warming.
The impact of list age
When migrating to a new Email Service Provider (ESP) or warming a new IP, using old engagement data can backfire. If your client's prior ESP contract ended months ago, their most recent engagement data might still be stale. This can lead to increased soft bounces from full inboxes and higher unsubscribe rates from recipients who no longer remember opting in.
Old or unengaged lists
Using contact lists with stale or outdated engagement data can quickly harm your IP reputation during warming. Gmail will see these as low-quality sends, triggering filters.
Increased soft bounces: More emails returned due to full mailboxes, indicating inactive users.
Higher unsubscribe rates: Recipients who don't remember opting in will frequently unsubscribe.
Spam complaints: Dissatisfied users may resort to marking as spam, severely damaging reputation.
Clean and engaged lists
Prioritizing recipients with recent engagement ensures a positive start to your IP warming. This strategy provides strong, positive signals to ISPs.
Manageable unsubscribe rates: Those who opt out do so via the unsubscribe link, not spam reports.
Strong positive engagement: High opens, clicks, and replies boost your sender reputation.
Remember, the primary goal of IP warming is to build a positive sending history. Any signals that indicate low engagement, poor list quality, or unwanted mail, whether it's soft bounces or unsubscribes, will hinder this process and can lead to a damaged sender reputation. Consistent attention to these metrics will help ensure your emails land in the inbox.
Views from the trenches
Best practices
Always prioritize sending to your most engaged subscribers first during any IP warming phase.
Implement clear and easy-to-find unsubscribe options, including List-Unsubscribe headers, to reduce spam complaints.
Continuously monitor your bounce and unsubscribe rates and adjust your sending volume or list segmentation accordingly.
Regularly clean your email lists to remove inactive addresses and ensure ongoing list hygiene.
Common pitfalls
Using outdated or old engagement data for IP warming, leading to unexpected soft bounces and unsubs.
Not having a clear unsubscribe process, which can lead to higher spam complaint rates instead of opt-outs.
Ignoring temporary soft bounces, especially 'mailbox full' errors, as they can signal underlying list quality issues.
Sending to a broad, unsegmented list during the early stages of IP warming, rather than focusing on engaged users.
Expert tips
If you have an old list, consider re-engagement campaigns before full warming to identify active subscribers.
A high unsubscribe rate, while not ideal, is generally preferred by ISPs over a high spam complaint rate.
Users with full mailboxes often indicate neglected accounts and a lack of recent engagement, so target carefully.
Gmail's Postmaster Tools provide valuable insights into your sender reputation, including bounce and spam rates.
Expert view
Expert from Email Geeks says that high soft bounces due to full inboxes usually indicate that the data being used is not the cleanest and requires re-evaluating the segmentation strategy.
2022-01-12 - Email Geeks
Marketer view
Marketer from Email Geeks says that they are surprised many people think sending to full mailboxes doesn't affect reputation, as they always thought repeated attempts could be seen as bad behavior.
2022-01-12 - Email Geeks
Ensuring a positive IP warming outcome
Successfully warming an IP address for Gmail (and other ISPs) requires a meticulous approach to list quality and engagement. While soft bounces and unsubscribe rates may not always lead to direct penalties, they serve as crucial indicators of your sending practices and list health. Maintaining a clean, engaged list and offering clear unsubscribe options are fundamental practices that pave the way for a strong sender reputation and optimal deliverability.
By actively managing these metrics and responding to the signals ISPs send, you can navigate the complexities of IP warming with greater confidence, ensuring your messages consistently reach the inbox.
Gmail is magnified. It's like building a reputation from scratch, where every interaction counts. Two crucial metrics that often cause concern are soft bounces and unsubscribe rates. Understanding how these impact your sender reputation, especially with Gmail, is key to a successful IP warming strategy.
Gmail looks at overall engagement. If you're consistently sending to mailboxes that are full, it suggests you're targeting inactive or neglected accounts. This can indirectly affect your sender reputation by indicating poor list hygiene or an unengaged audience, which ISPs dislike.
Mailbox Providers (MBPs) like Gmail than a spam complaint.
ISPs. Both soft bounces and unsubscribe rates, when poorly managed, can send negative signals. A common pitfall is using outdated or unengaged lists for warming, especially if there's been a long gap in sending.
