How to resolve 550 5.7.1 email blocklist errors when sending from 17hats?
Matthew Whittaker
Co-founder & CTO, Suped
Published 5 Jun 2025
Updated 16 Aug 2025
8 min read
Receiving a 550 5.7.1 email blocklist error when sending important client communications can be incredibly frustrating. This specific error message, such as "Unfortunately, messages from [66.96.185.10] weren't sent. Please contact your Internet service provider since part of their network is on our block list (S3150)," indicates that the receiving mail server, often Microsoft Outlook, has rejected your email because the sending IP address is on their internal or a third-party blocklist.
For users of platforms like 17hats, which serve as a CRM and email sending tool, these issues often arise not from individual sending practices, but from the broader reputation of the shared IP addresses used by the platform's underlying infrastructure. Even if you send low volumes of legitimate email, you can be affected by other users' poor sending habits on the same shared IP.
Understanding the precise nature of this block and how it relates to 17hats is crucial for effective resolution. The key lies in identifying who controls the sending IP and what steps need to be taken to get it delisted. This guide will walk you through diagnosing the problem and navigating the resolution process.
A common indicator in these bounce messages is the "Reporting-MTA" header, which often points to the actual mail transfer agent (MTA) responsible for the delivery attempt. For 17hats users encountering this error, you might see references to domains like bosmailout10.eigbox.net which is a critical piece of information for troubleshooting.
The 550 5.7.1 error is a general rejection code that signals the recipient's server has blocked your email. This block can occur for various reasons, but when accompanied by a blocklist (or blacklist) reference, it almost always points to a sender reputation issue with the sending IP address or domain. Mailbox providers like Outlook (Microsoft) maintain their own internal blocklists and subscribe to external ones to protect their users from spam and abuse.
When you send emails through a platform like 17hats, your messages are typically routed through their email infrastructure, which often involves using shared IP addresses. These IPs are used by many different users, meaning the sending reputation of the IP is an aggregate of all users' sending behaviors. Even if your personal email practices are stellar, the actions of other users on that shared IP can lead to its inclusion on a blocklist (or blacklist).
This situation highlights a fundamental challenge with shared IP environments. While cost-effective and suitable for low-volume senders, they carry the inherent risk of collateral damage from other senders. A sudden surge in spam, phishing, or even a large number of complaints from another user on the same IP can quickly lead to the IP being flagged and blocked by major mailbox providers, affecting everyone sharing that IP.
Understanding 17hats' sending paths and eigbox.net
The bounce message you receive contains critical clues, particularly the "Reporting-MTA" line. In the case of 17hats and this specific error, seeing bosmailout10.eigbox.net (or similar eigbox.net references) indicates that the emails are being sent through a network associated with Endurance International Group (EIG). EIG is a large web hosting company that owns many brands, and its networks often have a compromised reputation due to a high volume of problematic email traffic.
This presents two primary scenarios for how your email might be routing through eigbox.net:
In this case, 17hats itself might be utilizing EIG's network as its default email sending infrastructure for its users. If this is true, then the issue is entirely on 17hats' side.
It means their chosen email service provider or their direct setup has a poor reputation that is affecting all users on those shared IPs. Your individual email volume or content would not be the primary driver of the block.
Scenario 2: You've configured 17hats to use an external SMTP
It's possible that when setting up 17hats, you configured it to send emails through an external SMTP server, perhaps one provided by your web host (which might be an EIG company like Netfirms or iPage). In this situation, the problem originates from your chosen SMTP relay, not 17hats directly.
The solution would involve changing this SMTP configuration within 17hats to use a more reputable email sending service.
The distinction between these two scenarios is critical because the path to resolution depends on where the issue lies. If 17hats is responsible for the poor sending reputation, you'll need them to address it. If it's your configuration, you'll need to make changes on your end.
Immediate troubleshooting and communication
The first and most important step is to contact 17hats support. Provide them with the full bounce message, including the error code, the blocked IP address, and the `Reporting-MTA` details. Explain that you are experiencing a 550 5.7.1 block due to their sending IP being on a blocklist. Ask them to investigate if they are indeed routing your emails through eigbox.net and to take steps to delist the affected IPs.
Microsoft delisting process
While you can submit a delisting request to Microsoft's delisting form yourself, it's often more effective if the Internet Service Provider (ISP) or email service provider (ESP) who owns the IP address does it. They have direct communication channels and can provide the necessary context to Microsoft.
If you confirm that your 17hats account is configured to use an external SMTP server associated with eigbox.net, your immediate action should be to change this configuration. Switch to a reputable email service provider for your transactional and client communications. This might involve using 17hats' default sending if it is not problematic, or integrating with a dedicated transactional email service.
Always ensure that your domain's SPF and DKIM records are correctly configured and passing, even if the IP is the primary issue. These authentication methods are fundamental for demonstrating the legitimacy of your emails and will support any delisting efforts.
Proactive measures for sustained deliverability
For long-term email deliverability, especially when using shared IP addresses, proactive management is essential. Regularly monitor your sender reputation and look out for any signs of blocklisting. This includes using a blocklist checker to identify if your sending IP or domain appears on any major public or private blacklists. Staying vigilant can help you address issues before they escalate.
If you frequently encounter blocklist issues with your current setup, it might be time to evaluate whether your email service provider (ESP) is the right fit for your needs. A good ESP will actively manage their IP reputation, remove abusive senders, and provide clear paths to resolution for deliverability issues. This is crucial for avoiding recurring problems and maintaining consistent inbox placement.
Consider implementing and monitoring DMARC. While SPF and DKIM authenticate your emails, DMARC provides a policy layer and crucial reporting that helps you understand how your emails are being treated by recipient servers. This visibility is invaluable for diagnosing and resolving ongoing deliverability challenges.
Maintaining a clean email list, avoiding sending to invalid or disengaged addresses, and providing clear unsubscribe options will significantly contribute to a positive sender reputation. These practices reduce bounce rates and spam complaints, which are major factors in IP and domain blocklisting. Remember, good sending practices are always the foundation of successful email deliverability.
Views from the trenches
Best practices
Actively communicate with your email service provider to ensure they are aware of and addressing blocklist issues on shared IPs.
Always include the full bounce message when reporting delivery errors to your provider, as it contains vital diagnostic information.
Regularly review your email authentication records (SPF, DKIM, DMARC) to ensure they are correctly configured and passing.
Maintain a clean and engaged email list to reduce spam complaints and bounce rates, improving overall sender reputation.
Consider segmenting your email sends to different IPs if your provider allows, separating transactional from marketing emails.
Common pitfalls
Assuming that low personal sending volume means you cannot be affected by shared IP blacklists.
Neglecting to provide your email service provider with the specific error message and blocked IP address.
Attempting to delist shared IPs yourself without the cooperation or knowledge of the IP owner (your ESP).
Failing to investigate the underlying sending infrastructure used by your platform, which can hide reputation problems.
Ignoring the root causes of recurring blocklist issues and only seeking temporary fixes for individual incidents.
Expert tips
If your ESP uses an IP range that frequently appears on public blacklists, inquire about migrating to a more reputable pool of IPs.
Use email header analysis tools to trace the full path of your emails and identify any unexpected MTAs like eigbox.net.
For critical transactional emails, consider a dedicated IP if your volume warrants it, or switch to an ESP with superior shared IP management.
Regularly check your domain's sending reputation using tools like Google Postmaster Tools for Gmail delivery insights.
Understand that delisting an IP is only a temporary fix if the underlying bad sending practices on that shared IP continue.
Marketer view
A marketer from Email Geeks says that if you're on shared IP addresses, the email service provider (ESP) like 17hats needs to open the ticket with Microsoft, as the block is likely due to other clients on the shared space causing issues.
Jan 25, 2022 - Email Geeks
Expert view
An expert from Email Geeks says that mail coming out via eigbox.net, associated with Endurance, often has a poor reputation, suggesting it's the root of the problem regardless of how it's configured.
Jan 25, 2022 - Email Geeks
Ensuring your emails reach the inbox
Resolving 550 5.7.1 email blocklist errors when sending from 17hats requires a clear understanding of your email's sending path and a collaborative approach with your email service provider. The repeated occurrence of these errors, especially when referencing domains like eigbox.net, points to deeper reputation issues with the underlying sending infrastructure.
Your primary course of action should be to engage 17hats support, providing them with all necessary details to investigate and resolve the blocklist entry on their shared IP addresses. If it's discovered that your account is configured to use an external, poorly reputed SMTP server, adjust your 17hats settings to use a more reliable sending service.
Beyond immediate fixes, adopting proactive email deliverability practices, such as consistent monitoring and maintaining list hygiene, will fortify your email program against future blocklist challenges. By understanding the intricacies of email routing and advocating for your sender reputation, you can ensure your important client communications consistently reach the inbox.
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Outlook (Microsoft) maintain their own internal blocklists and subscribe to external ones to protect their users from spam and abuse.
