Should I resend emails that hard bounced during the Gmail outage?
Michael Ko
Co-founder & CEO, Suped
Published 22 Jun 2025
Updated 15 Aug 2025
7 min read
When a major email service provider experiences an outage, it can send ripples through the email marketing world. One particularly vexing issue that arises is the question of how to handle hard bounces received during such events. Typically, a hard bounce signifies a permanent delivery failure, like an invalid email address. However, during an outage, the lines can blur, leading to what appear to be hard bounces that are actually temporary issues.
The December 2020 Gmail outage serves as a prime example. Many senders reported a significant spike in hard bounces, specifically 5.1.1 errors, which usually indicate an unknown user. This situation raised a critical question: Should these emails be treated as genuine hard bounces and removed from lists, or should they be resent? Let's explore the nuances of this decision and provide guidance for future similar scenarios.
Understanding hard bounces and outages
A hard bounce fundamentally means that an email could not be delivered for a permanent reason. This could be because the email address is misspelled, no longer exists, or the domain itself is invalid. Typically, these addresses should be promptly removed from your mailing list to protect your sender reputation and avoid being flagged by ISPs. Repeatedly sending to hard bounced addresses can lead to your IP or domain being added to a blacklist, impacting your overall email deliverability.
However, during a widespread outage affecting a major email provider (like Gmail), the situation changes. The bounces you receive might not indicate an invalid address but rather a temporary inability of the recipient server to process the incoming mail. This is why it's crucial to understand the context of the bounce. A 5.1.1 error (Bad Destination Mailbox Address) normally implies a non-existent user, but if it occurs en masse during a confirmed outage, it's likely a misdiagnosis by the sender's system or the recipient's temporarily overwhelmed server.
In fact, during the December 2020 outage, Google itself acknowledged the issue and even recommended that users resend emails that had bounced to valid @gmail.com addresses. This is a rare directive, underscoring that not all hard bounces are created equal, especially when external factors like outages are at play. You can also refer to this article on erroneous hard bounces.
Identifying false positive hard bounces
The key to managing bounces during an outage is careful analysis. Don't immediately assume every hard bounce from that period indicates an invalid address. Instead, look for patterns specific to the outage. Were the bounces concentrated on a single domain or across various domains of the affected provider? Did the bounce messages explicitly state a server issue or temporary unavailability, even if the bounce code was typically associated with permanent errors?
Many email service providers (ESPs) will automatically categorize bounces and suppress future sends to hard-bounced addresses. However, during an outage, this automatic suppression might remove valid recipients from your list. It is crucial to dive into your bounce logs for the specific dates of the outage, such as the period of Gmail bounce issues, to manually review them. Look for common bounce codes like 5.1.1 or 550 that were unusually high during the outage period.
A soft bounce, indicating a temporary issue like a full mailbox, typically allows for retries. However, when server-side issues cause hard bounces, these generally aren't automatically retried. Therefore, it's up to the sender to determine if a hard bounce definition applies or if a retry is warranted. This involves carefully segmenting your bounce list to isolate the affected addresses during the outage window. You can segment based on the bounce reason, the error code, and the timestamp of the bounce.
Identifying outage-related bounces
Timeframe: Focus on bounces that occurred precisely during the reported outage period.
Bounce codes: Pay close attention to 5.x.x codes, especially 5.1.1 (user unknown), if they are unusually prevalent.
Recipient domain: Filter for bounces from the affected domain, such as @gmail.com.
Bounce message: Read the accompanying message, as it might contain context about temporary server issues or overloaded systems.
The risks and rewards of resending
Resending to addresses that genuinely hard bounced is detrimental to your email program. It signals to ISPs that you are not managing your lists effectively, which can lead to lower inbox placement, increased spam complaints, and even inclusion on a blocklist (or blacklist). However, in the unique context of an outage, resending to false positive hard bounces can be beneficial. It allows you to reach subscribers who would have otherwise been mistakenly suppressed.
The potential reward lies in recovering missed engagement and sales opportunities. During a significant outage, a considerable portion of your audience might not have received your important communications. By carefully identifying and resending to those affected by the outage, you can ensure they receive your message, maintaining a healthier relationship with your subscribers.
Risk of resending indiscriminately
Sender reputation damage: Repeatedly hitting non-existent addresses will harm your standing with ISPs.
Blocklist inclusion: A high bounce rate, especially hard bounces, can lead to your IP or domain being added to a blocklist.
Wasted resources: Sending to invalid addresses consumes bandwidth and can incur costs with some ESPs.
Opportunity of targeted resends
Recover engagement: Reach subscribers who genuinely wanted your email but missed it due to the outage.
Maintain list hygiene (indirectly): Prevents the mistaken removal of active subscribers from your list.
Improved accuracy: Your email performance metrics will be more accurate by excluding outage-induced anomalies.
Best practices for resending after an outage
If you decide to resend, a conservative and targeted approach is best. First, isolate the specific email addresses that hard bounced during the outage and belong to the affected provider. Do not resend to any address that had previously hard bounced outside of the outage window, as those are likely true invalid addresses. For addresses that show the 5.1.1 error on the specific outage day, consider them candidates for resending. Before resending, confirm that the outage has passed and the provider's services have stabilized.
When you do resend, monitor your deliverability metrics closely. Look for successful deliveries, open rates, and any new bounces from this segment. If the resend also results in hard bounces for the same addresses, then it's a strong indication that the address is indeed invalid and should be removed from your list permanently. This continuous monitoring is part of effective hard bounce management.
Bounce type
Typical action
Action during outage
True hard bounce
Immediately remove from list.
Still remove, regardless of outage.
Soft bounce (temporary)
ESPs usually retry automatically; monitor status.
Likely resolves after outage; still monitor.
Outage-induced hard bounce (e.g., 5.1.1 from Gmail during outage)
Normally remove from list.
Review logs and resend to valid addresses after outage. Monitor results.
Final considerations
Ultimately, the decision to resend emails that hard bounced during a Gmail (or any major provider) outage depends on a thorough analysis of your bounce logs and the specifics of the outage. While standard practice dictates immediate removal of hard bounced addresses, an outage introduces an anomaly where temporary server issues can mimic permanent failures.
By segmenting your affected audience and resending only to those genuinely impacted by the outage, you can recover potentially lost engagement without jeopardizing your overall email deliverability and sender reputation. Always prioritize careful data-driven decisions over blanket actions to maintain a healthy email program.
Views from the trenches
Best practices
Review bounce logs for the specific date and time of the outage to identify affected email addresses.
Segment recipients by domain (e.g., @gmail.com) and bounce reason (e.g., 5.1.1) during the outage.
Consult official post-mortem reports from the email provider for resend recommendations.
Common pitfalls
Automatically removing all hard bounces from the outage period without further investigation.
Resending to addresses that previously hard bounced outside of the outage window.
Ignoring bounce messages and only focusing on the numerical bounce codes.
Expert tips
Examine the SMTP response messages for explicit indications of temporary server issues, not just the generic code.
If possible, use email validation services that offer 'risky' or 'unknown' categories to re-check outage bounces.
Implement robust bounce processing that differentiates between permanent and transient failures, especially for major ISPs.
Expert view
Expert from Email Geeks says: It is advisable to resend emails that hard bounced during the Gmail outage, as many of these were false positives caused by server issues.
December 14, 2020 - Email Geeks
Expert view
Expert from Email Geeks says: Revalidating all Gmail bounces from that specific outage period, not just the 5.1.1 errors, is effective because any truly invalid addresses will simply bounce again later.
Gmail), the situation changes. The bounces you receive might not indicate an invalid address but rather a temporary inability of the recipient server to process the incoming mail. This is why it's crucial to understand the context of the bounce. A 5.1.1 error (Bad Destination Mailbox Address) normally implies a non-existent user, but if it occurs en masse during a confirmed outage, it's likely a misdiagnosis by the sender's system or the recipient's temporarily overwhelmed server.
Gmail during outage)
