What spam filters do NetZero and Juno use, and how can I resolve blocking issues?
Michael Ko
Co-founder & CEO, Suped
Published 10 Jul 2025
Updated 15 Aug 2025
6 min read
Dealing with email blocking issues from internet service providers (ISPs) can be a significant challenge, especially when trying to maintain a strong sender reputation. NetZero and Juno, while perhaps not as prominent as some larger email providers today, still serve a substantial user base, and deliverability to their inboxes remains crucial for many senders. I've navigated these waters many times, and understanding their unique spam filtering mechanisms is the first step toward resolving any blocking issues you might encounter.
When you're engaged in IP warming, encountering blocks from these providers can be particularly frustrating, even if your mail streams are performing well elsewhere with minimal bounces and engaged subscribers. It suggests their filtering is more sensitive or operates on different criteria than other mailbox providers. Let's explore what their filters entail and how to troubleshoot common issues.
How NetZero and Juno handle spam
NetZero and Juno, both operating under United Online, primarily employ their own proprietary, homegrown spam filters. Unlike some larger email providers that might rely heavily on third-party solutions, these ISPs have developed their internal systems over time. Their approach focuses on a combination of automated detection and user-defined preferences to manage unwanted email.
Junk mail filter: NetZero's Junk Mail Filter, for instance, is designed to detect and block a significant percentage of spam before it ever reaches the user's inbox, claiming to catch over 90% of junk mail. Users can configure these spam filter settings to suit their needs.
Block list: Both NetZero and Juno provide a block list feature, allowing users to manually add specific email addresses or entire domains they wish to block. This is often used for known spammers or unwanted newsletters.
Safe list: Conversely, a Safe List (or whitelist) lets users designate trusted senders, ensuring that their emails always reach the inbox.
These filtering mechanisms, while effective for users, can sometimes lead to legitimate emails being caught in the spam filter. The filters evolve based on user feedback and observed spam patterns, making it essential for senders to maintain a high reputation and adhere to best practices.
Identifying blocking issues
Identifying that your emails are being blocked by NetZero or Juno typically involves reviewing your bounce reports. You'll often see specific error messages that indicate a policy or spam-related block. A common indicator is an "Access denied" error transcript, which signifies that NetZero's anti-spam filters have blocked the email. Another possible bounce code is the "5.3.2 soft bounce" error, which can mean your mail server's IP address is being rejected due to perceived suspicious activity or policy violations.
Understanding block indicators
When an email is blocked by an ISP, the bounce report usually provides clues. For NetZero and Juno, these often appear as policy-related or spam-related bounces. Pay close attention to the full error message, as it can pinpoint the exact reason for the block. For more details on this specific error, you can refer to our guide on 5.3.2 soft bounce errors.
This message explicitly directs senders to a specific postmaster page, which is crucial for initiating the resolution process. It's a clear signal that your IP address has been added to their internal blocklist or is otherwise triggering their filters.
Early stages of IP warming are particularly vulnerable because these ISPs are evaluating your sending patterns and reputation. Any sudden increase in volume, even to engaged recipients, can be misinterpreted as suspicious activity, leading to temporary or permanent blocks (also called blacklists or blocklists).
Strategies to resolve blocking issues
The most direct way to address blocks with NetZero and Juno is to use their designated postmaster site. This is where you can find information on their policies and submit requests for delisting (removal from a blocklist). The URL for NetZero and Juno's postmaster page is http://postmaster.untd.com/blocked.html. Submitting a request through this form is often the most effective way to communicate with their deliverability team and initiate a review of your sending practices.
Beyond direct contact, ensuring your email authentication is correctly configured is paramount. NetZero and Juno, like most ISPs, rely heavily on SPF, DKIM, and DMARC to verify sender legitimacy. Incorrect or missing records can significantly impact your deliverability. For a comprehensive overview, review our guide on DMARC, SPF, and DKIM.
Proactive measures
Authentication: Ensure your SPF, DKIM, and DMARC records are correctly set up and aligned.
List hygiene: Regularly clean your mailing lists to remove inactive users, bounces, and known spam trap addresses.
Engagement: Segment your lists and send to your most engaged subscribers first. This builds positive sender reputation.
Bounce analysis: Thoroughly analyze bounce messages for specific error codes or explanations from NetZero/Juno.
Content review: If bounces indicate content issues, review your email for common spam triggers like excessive links, suspicious phrasing, or poor formatting.
Volume reduction: Temporarily reduce sending volume to NetZero and Juno domains if you observe persistent blocking, then gradually increase it.
Remember that IP warming is a delicate process, and even minor issues can lead to blocks from sensitive ISPs like NetZero and Juno. Consistent, positive sending practices are key to building and maintaining trust.
Maintaining good sender reputation
Your sender reputation is critical for deliverability to any ISP, including NetZero and Juno. This reputation is built over time based on various factors, including bounce rates, spam complaint rates, subscriber engagement, and adherence to email sending best practices. Regularly monitoring your reputation is a proactive measure that can prevent future blocking issues and help you recover more quickly from existing ones.
Factor
Impact on deliverability
Action to take
Bounce rate
High bounce rates indicate a poor list quality or potential block. Soft bounces can become hard bounces if unaddressed.
Regularly clean your lists and remove invalid or inactive addresses.
Spam complaints
A high complaint rate signals that recipients don't want your emails, leading to blocklists (or blacklists) and spam folder placement.
Provide clear unsubscribe options and send relevant content to engaged users.
Engagement
Low open or click rates, combined with high deletions or complaints, negatively impact your sender reputation.
Focus on content that resonates with your audience and segment for better targeting.
Blocklist status
Being listed on a public or private blocklist (or blacklist) can lead to widespread delivery issues across multiple ISPs.
While NetZero and Juno may use their own internal filters, they can also be influenced by larger, shared blocklists or spam filtering services. Staying vigilant with your overall email program, including blocklist monitoring, and maintaining a strong domain reputation, is key to successful deliverability across all providers. For more information on how your domain's reputation affects email delivery, check out our guide on understanding your email domain reputation.
Views from the trenches
Best practices
Always prioritize sending to your most engaged subscribers, especially during IP warming phases.
Use the specific postmaster forms provided by NetZero and Juno for delisting requests.
Maintain strict list hygiene by removing unengaged subscribers and invalid email addresses.
Ensure all email authentication protocols (SPF, DKIM, DMARC) are correctly implemented.
Common pitfalls
Assuming NetZero and Juno use common third-party spam filters instead of their own proprietary systems.
Ignoring bounce messages that contain
Access denied
or
Expert tips
It's crucial to understand that even older ISPs like NetZero and Juno have robust, often homegrown, filtering mechanisms that require specific attention.
While a centralized postmaster contact list would be ideal, it's essential to maintain your own updated records for all key ISPs.
If your IPs are being blocked, and you suspect Cloudmark's involvement, direct engagement with their team can clarify the situation.
Proactive list management and authenticating your emails are the strongest defenses against being blocked.
Expert view
Expert from Email Geeks says that NetZero and Juno primarily utilize their own homegrown email filters for spam detection.
2020-09-11 - Email Geeks
Expert view
Expert from Email Geeks says that NetZero and Juno are not tiny ISPs, but rather older ones that still maintain a significant user base.
2020-09-11 - Email Geeks
Summary of deliverability to NetZero and Juno
Successfully delivering emails to NetZero and Juno (United Online) requires a nuanced approach, recognizing their reliance on homegrown spam filters and specific postmaster communication channels. While they may not use widely known third-party blocklists, their internal systems are sensitive to sender reputation, authentication, and content relevance.
By actively monitoring your bounce reports, utilizing their postmaster form, ensuring proper email authentication, and maintaining robust list hygiene, you can mitigate blocking issues and improve your overall deliverability to these important email providers.
United Online, primarily employ their own proprietary, homegrown spam filters. Unlike some larger email providers that might rely heavily on third-party solutions, these ISPs have developed their internal systems over time. Their approach focuses on a combination of automated detection and user-defined preferences to manage unwanted email.
