What does a 5.3.2 soft bounce error code mean when sending emails to Juno and NetZero?
Michael Ko
Co-founder & CEO, Suped
Published 26 Apr 2025
Updated 19 Aug 2025
6 min read
Recently, I've seen discussions about the 5.3.2 SMTP error code, especially when sending emails to Juno and NetZero addresses. This specific error, often accompanied by the message "system not accepting network messages", indicates a temporary delivery failure, classifying it as a soft bounce.
While soft bounces are generally temporary issues, encountering a consistent stream of 5.3.2 errors, particularly with specific domains like Juno and NetZero, suggests there might be underlying problems that warrant closer investigation. It's crucial to understand what this error truly signifies for your email deliverability.
Decoding the 5.3.2 soft bounce error
The 5.3.2 error code falls into the 5xx range, which typically denotes a permanent failure. However, in this specific context, and particularly with some older email providers, it functions more like a soft bounce. This means the recipient's mail server encountered an issue that prevented immediate delivery, but it might not be a permanent rejection of the email address itself.
Common reasons for a 5.3.2 error can include the recipient's mail system being temporarily overloaded, undergoing maintenance, or experiencing an unplanned shutdown. It essentially means the server is unable to process the incoming message at that specific moment. Unlike a hard bounce, which signifies an invalid address or permanent block (blacklist), a soft bounce suggests a transient problem. More information on email bounce codes can be found in our guide to understanding what they mean.
While temporary in nature, repeated 5.3.2 errors from the same recipient or domain are a red flag. They can indicate deeper issues like an IP block (or blacklist), excessive sending volume, or a general poor sender reputation with that specific receiving server. It's important to differentiate between an isolated incident and a systemic pattern.
Soft vs. hard bounces
Soft bounces: Temporary delivery failures due to full mailboxes, server downtime, or large messages. Sending servers usually retry.
Hard bounces: Permanent failures from invalid email addresses or non-existent domains. These must be immediately removed.
The unique challenges of Juno and NetZero
Juno and NetZero are older internet service providers (ISPs) that have historically served a user base primarily reliant on dial-up internet. Their email infrastructure may not be as robust or as updated as that of larger, more modern email providers. This can lead to unique deliverability challenges.
These legacy ISPs often employ filtering mechanisms that can be less transparent or more sensitive to certain sending patterns, even if your overall sending practices are sound. They might have lower tolerance thresholds for incoming mail, leading to temporary rejections like the 5.3.2 code, even without explicit blocklisting (or blacklisting).
Furthermore, the user base for Juno and NetZero may have different email usage habits, potentially leading to higher rates of inactive accounts or less frequent mailbox checks. This can inadvertently contribute to bounce issues if your list isn't regularly cleaned of less engaged subscribers.
Legacy ISPs (Juno & NetZero)
Older infrastructure and potentially less scalable systems.
Filtering mechanisms can be less transparent, leading to unexpected bounces.
Modern ISPs
Google and Outlook have robust, scalable infrastructure.
They offer clearer postmaster tools and feedback loops.
Diagnosing and addressing the 5.3.2 error
When faced with consistent 5.3.2 errors, especially specific to domains like Juno and NetZero, the first step is to verify your sender reputation. Even if you're not on a public blocklist (blacklist), an ISP can internally block (or blacklist) your IP or domain.
Checking the postmaster pages for these specific ISPs, if available, can sometimes provide insights into their policies or any active blocks (or blacklists). For example, Juno and NetZero are part of the United Online network, which has a postmaster site at postmaster.untd.com. This resource might offer information on specific block issues. It's also important to understand the difference between a blacklist and a blocklist.
If you're sending through an Email Service Provider (ESP), confirm whether you're using a shared IP pool or a dedicated IP address. Shared IP pools can be impacted by the sending behavior of other users on the same pool, potentially affecting your reputation with certain ISPs. If you suspect an IP reputation issue, it's useful to look at ways to improve your domain reputation.
Review your email authentication records, including SPF, DKIM, and DMARC. Proper configuration of these protocols signals to receiving mail servers that your emails are legitimate and can significantly influence deliverability. A simple guide to DMARC, SPF, and DKIM can help you ensure these are set up correctly.
Strategies for improving deliverability to challenging domains
The most effective long-term strategy for mitigating soft bounces, including 5.3.2 errors, is rigorous list hygiene. Regularly remove inactive or unengaged subscribers, and immediately cease sending to addresses that consistently hard bounce. This practice helps maintain a healthy sender reputation.
If the volume of bounces to these specific domains is relatively small, as was observed in the initial scenario (68 out of 24,400), it might be more pragmatic to simply remove those addresses from your mailing list. The effort required to troubleshoot and maintain deliverability to very small, challenging legacy ISPs might not yield a worthwhile return. This approach can also improve your overall email deliverability.
Implement re-engagement campaigns for older or less active segments of your list. Subscribers who haven't opened or clicked an email in a long time may have abandoned their addresses, turning them into potential spam traps or simply inactive recipients that lead to bounces.
Monitor your sending practices closely. Ensure your content is relevant and engaging, avoid spammy triggers, and maintain a consistent sending volume. Gradual IP warming for new IPs or significant changes in sending patterns can also prevent sudden blocks (or blacklists). You can also run an email deliverability test to see how your emails perform.
Best practices for problematic domains
List hygiene: Regularly clean your email lists to remove inactive and bouncing addresses.
Authentication: Ensure SPF, DKIM, and DMARC are correctly implemented for trust.
Consistency: Maintain consistent sending volumes to avoid triggering spam filters.
Navigating legacy ISP deliverability
The 5.3.2 soft bounce error, particularly from Juno and NetZero, often signals temporary system issues or challenges with older email infrastructure. While not always an immediate indictment of your sending practices, consistent occurrences warrant attention to protect your sender reputation.
By focusing on strong email authentication, rigorous list hygiene, and understanding the nuances of how these specific ISPs operate, you can improve your overall deliverability. Sometimes, the most pragmatic solution for marginal domains is to strategically clean them from your active lists.
Views from the trenches
Best practices
Actively monitor all bounce messages, not just the hard bounces, to catch early signs of temporary issues that could escalate.
Segment your email lists by domain to identify and address deliverability issues specific to certain ISPs like Juno or NetZero.
Prioritize email authentication (SPF, DKIM, DMARC) as a foundational element of your deliverability strategy to build sender trust.
Common pitfalls
Ignoring soft bounces, assuming they will resolve on their own, which can lead to larger deliverability problems over time.
Not removing inactive addresses, which can lead to higher bounce rates and signal poor list management to ISPs.
Sending inconsistent volumes or content, which can trigger spam filters and result in temporary blocks (or blacklists).
Expert tips
For very small or legacy domains that consistently cause bounces, it may be more efficient to remove those addresses rather than dedicate significant resources to troubleshooting.
A 5.3.2 error often indicates a temporary server-side problem at the recipient's end, rather than a direct sender reputation issue.
Always check specific ISP postmaster pages for direct insights, even if general blocklists don't show a listing.
Expert view
Expert from Email Geeks says 5.3.2 errors typically indicate an IP reputation problem with Juno or NetZero, suggesting a mail block.
2020-02-18 - Email Geeks
Expert view
Expert from Email Geeks says Juno usually uses a 550 access denied code for blocks, making a 5.3.2 error less indicative of a direct block and more possibly a system issue.
Google and 
Outlook have robust, scalable infrastructure.
