What causes 4003 and 4004 Gmail soft bounce errors and how to resolve them?
Michael Ko
Co-founder & CEO, Suped
Published 12 Jun 2025
Updated 16 Aug 2025
7 min read
Dealing with email bounce errors can be a frustrating experience, especially when you are trying to ensure your important messages reach their intended recipients. While hard bounces indicate a permanent delivery failure, soft bounces are temporary. They signal that an email couldn't be delivered on the first attempt, but the mail server will try again. However, when these soft bounces appear with specific Gmail error codes like 4003 and 4004, it can be particularly perplexing. Understanding what these codes mean and how to address them is crucial for maintaining your email deliverability.
These aren't standard SMTP response codes, which can make them difficult to interpret. They are often internal error codes used by specific mail transfer agents (MTAs) or email service providers (ESPs) to provide more granular detail about a temporary delivery issue when sending to Gmail addresses. I'll explain what these errors signify and the steps you can take to resolve them to ensure your emails reliably reach Gmail inboxes.
Decoding Gmail's soft bounce codes
When an email experiences a soft bounce, it means the message reached the recipient's mail server, but it was not accepted for delivery to the inbox at that moment. Unlike a hard bounce, which indicates a permanent problem such as an invalid address, a soft bounce suggests a temporary issue that might resolve itself. Common reasons include a full inbox, server downtime, or the message being too large. Gmail's 4003 and 4004 errors fall into this category, but their specifics point to network and system-level issues rather than typical mailbox problems.
Understanding 4003 network error
The 4003 Network Error, when encountered with Gmail, usually points to issues related to the network connectivity between the sending server (your ESP) and Gmail's servers. These are generally transient problems that are out of your direct control as a sender. My experience suggests these are often due to:
Server down: The recipient's mail server might be temporarily offline or undergoing maintenance.
Routing issues: There could be a temporary glitch in the routing of network packets, preventing the email from reaching Gmail's servers.
Understanding 4004 other receiver error
The 4004 Other Receiver Error is a broader category, but still points to a temporary problem on the receiving end. While less specific than 4003, it often implies an issue within Gmail's internal systems or network infrastructure. Based on typical ESP interpretations, this can cover:
DNS error: Problems with DNS resolution preventing your ESP from finding Gmail's mail servers.
Internal network error: A temporary issue within Gmail's own network.
External network error: Similar to 4003, but perhaps originating from an external component that Gmail relies on.
Both 4003 and 4004 errors typically do not indicate a problem with your sender reputation or email content being flagged as spam. Instead, they are more indicative of momentary technical hurdles in the email transmission process. Many 4xx errors are transient, and a retry mechanism by the sending server usually resolves them.
Common causes behind 4003 and 4004 errors
While 4003 and 4004 errors are usually temporary and indicate network or server issues, a consistent pattern of these errors could suggest a deeper underlying problem. It's important to differentiate these from other common soft bounces, such as a full inbox. Here's a quick comparison:
4003/4004 Gmail specific soft bounces
Primary cause: Temporary network or internal server problems at Gmail.
Reputation impact: Usually minimal, as it's not tied to spam filtering or content.
Resolution: Automated retries by ESP are often sufficient.
Other common soft bounces
Primary cause: Recipient inbox full, message too large, or temporary content filtering.
Reputation impact: Can sometimes indicate issues with list hygiene or sending practices.
Resolution: May require list cleaning, segmentation, or content adjustments.
If you're seeing a significant spike in these Gmail-specific soft bounces, especially if you're a small volume sender using shared IPs, it could suggest that the shared IP pool used by your ESP might be experiencing temporary network issues or perhaps a momentary (not permanent) blocklisting event. While it’s rare for a major provider like Gmail to have sustained network issues, localized or temporary glitches can occur.
Another potential, though less common, cause could be a very brief but widespread DNS resolution problem between your ESP's servers and Gmail's domains, leading to an inability to connect. This would present as a 4004 error. These underlying network issues can sometimes stem from upstream providers or specific data center connectivity.
Diagnosing and troubleshooting these errors
The most crucial step in troubleshooting these errors is to gain as much information as possible. Unfortunately, many ESPs simplify bounce messages, providing generic codes like 4003 or 4004 without the underlying SMTP response. If your ESP provides full bounce logs, dig into those for the exact SMTP error code and message. This can offer a clearer picture, as a standard SMTP code might accompany these proprietary codes.
Even though these errors are often not reputation-related, it's always a good practice to monitor your sender reputation. Keep an eye on Google Postmaster Tools for any unusual dips in IP or domain reputation, or spikes in spam complaints. While unlikely to be the primary cause of 4003/4004, it provides a holistic view of your sending health. You can also monitor for your IP or domain getting on any major blacklists (or blocklists) even if it's a temporary listing. Sometimes a temporary blocklist (or blacklist) can manifest as a routing or network issue.
Here are some immediate diagnostic steps you can take:
Key diagnostic steps
Contact your ESP: Ask for the complete SMTP bounce message associated with the 4003/4004 errors. They have more detailed logs.
Check ESP status pages: Look for any reported outages or issues with their sending infrastructure or specific IP pools.
Review Postmaster Tools: Even if a minor dip, check your domain and IP reputation. Look at the Delivery Errors dashboard for more specific error types.
Strategies for resolution
Given that 4003 and 4004 errors are typically temporary network or system glitches, the primary resolution is often simply to allow your ESP to retry sending the message. Most reputable ESPs will automatically retry soft bounces over a period of time before converting them into hard bounces. If your ESP does not automatically retry, or if you manage your own sending infrastructure, implementing a retry mechanism is essential.
Beyond simply retrying, there are foundational email deliverability practices that, while not directly addressing transient network errors, contribute to a robust sending infrastructure and better overall inbox placement. These practices can help mitigate the impact of any underlying, subtle issues that might contribute to higher soft bounce rates.
Strategies for improved deliverability
Maintain clean lists: Regularly remove inactive or unengaged subscribers to reduce potential bounce rates, even soft ones. A healthy list contributes to better sender reputation.
Ensure proper authentication: Always have SPF, DKIM, and DMARC correctly configured. This helps Gmail verify your sending legitimacy and reduces the likelihood of any temporary blocks.
Monitor your deliverability: Utilize tools like Postmaster Tools and your ESP's analytics to track bounce rates and other key metrics. Understanding your trends can help identify issues quickly.
In summary
When encountering Gmail 4003 and 4004 soft bounce errors, it's crucial to understand their temporary nature and what they typically indicate. While frustrating, these errors are usually due to transient network or system issues, rather than problems with your content or sender reputation. Focusing on robust retry mechanisms and maintaining fundamental deliverability best practices will go a long way in ensuring your emails eventually reach their desired destination.
Views from the trenches
Best practices
Always check with your Email Service Provider (ESP) for specific interpretations of non-standard bounce codes like 4003 and 4004. They often map these to internal system statuses.
Implement robust retry mechanisms for soft bounces. Most ESPs handle this automatically, but confirm their retry policy and duration.
Maintain high sender reputation by sending relevant content to engaged subscribers and avoiding sudden spikes in sending volume.
Proactively monitor your domain and IP health using tools like Google Postmaster Tools, which can offer insights into any underlying issues impacting Gmail delivery.
Common pitfalls
Assuming 4003/4004 errors are reputation-related without proper investigation, leading to unnecessary and potentially harmful changes to your sending strategy.
Not having visibility into the full SMTP bounce message, which can obscure the true underlying cause of the temporary delivery failure.
Ignoring a persistent rise in soft bounces, even temporary ones, as it could indicate a creeping problem that might escalate into harder blocks.
Over-segmenting email lists or applying aggressive filtering based on temporary soft bounces, which might unnecessarily reduce your reach.
Expert tips
If your ESP provides limited bounce details, consider asking for direct access to SMTP logs for more granular troubleshooting data.
Focus on the long-term health of your sending program. Consistent authentication, good list hygiene, and relevant content are always the best defense against deliverability issues.
Shared IP pools can be volatile; if you're a small sender experiencing consistent issues, inquire about the reputation of the specific IP addresses your emails are being sent from.
Remember that Gmail often prioritizes network stability. These temporary errors could be a proactive measure to manage traffic or internal system load.
Marketer view
Marketer from Email Geeks says they observed a significant spike in Gmail soft bounces across multiple small-volume clients using the same ESP and shared IPs.
2021-05-24 - Email Geeks
Expert view
Expert from Email Geeks says that understanding the actual bounce message from Gmail is crucial for diagnosis.
for any unusual dips in IP or domain reputation, or spikes in spam complaints. While unlikely to be the primary cause of 4003/4004, it provides a holistic view of your sending health. You can also monitor for your IP or domain getting on any major blacklists (or blocklists) even if it's a temporary listing. Sometimes a temporary blocklist (or blacklist) can manifest as a routing or network issue.
