Why am I seeing 5.4.4 'no mail hosts' errors for Microsoft domains?
Matthew Whittaker
Co-founder & CTO, Suped
Published 25 Jun 2025
Updated 18 Aug 2025
6 min read
Encountering a 5.4.4 'no mail hosts' error when sending emails to Microsoft domains (like Hotmail, Outlook.com, Live, or MSN) can be incredibly frustrating. This non-delivery report (NDR) signals that the recipient's email server was unable to find a valid mail host for your domain, essentially failing to route your message to its intended destination.
While the error message points to a problem with the recipient's domain, the root cause often lies within the sender's email infrastructure, specifically related to DNS resolution, or it could be a transient issue on the recipient's network. Understanding these underlying issues is key to diagnosing and resolving the problem quickly.
The meaning of 5.4.4 'no mail hosts'
The 5.4.4 error code, often accompanied by messages like 'unable to route: no mail hosts for domain' or 'DNS error:NXDOMAIN', signifies a permanent delivery failure. Your sending mail server tried to perform a DNS lookup for the recipient's domain to find its Mail Exchanger (MX) records, but it failed to get a valid response.
For major providers like Microsoft, this indicates their mail servers could not resolve the necessary DNS information to route your email. While the error originates from the receiving end, the problem is often related to how your system (or its DNS resolver) interacts with their DNS infrastructure.
It's important to differentiate this from temporary errors, which usually start with a '4xx' code and suggest a transient issue. A 5.4.4 error implies a fundamental lookup failure, even if it might sometimes be caused by a momentary glitch that self-corrects.
Understanding the problem
DNS resolution failure: Your sending server cannot find the recipient domain's MX record.
Outdated cache: Your DNS resolver may have stale information.
Firewall interference: Network rules might be blocking DNS queries.
Decoding common causes
The primary cause for 5.4.4 errors often resides with the sending mail server's inability to correctly resolve the recipient domain's MX records. This can be due to a variety of factors on your end, such as an outdated DNS cache, restrictive firewall rules blocking outbound DNS queries, or a misconfigured DNS resolver on your sending infrastructure.
In rarer cases, the issue might stem from the recipient's actual MX records being temporarily unavailable or misconfigured, though this is uncommon for large-scale providers like Outlook.com. Temporary network congestion or DNS server outages affecting the lookup path can also trigger this bounce, often resolving themselves quickly.
For some mail transfer agents (MTAs) like PowerMTA, specific configurations, such as the edns-udp-length setting, can influence DNS query behavior and prevent these types of errors. Ensuring your MTA is configured to handle DNS responses correctly is important.
Practical troubleshooting steps
To begin troubleshooting, your first step should be to verify the recipient domain's MX records. You can use online DNS lookup tools to confirm that domains like Hotmail.com or Live.com have valid and resolvable MX entries. Concurrently, check your own mail server's DNS resolver configuration. Make sure it's pointing to reliable DNS servers and that there are no issues with Time To Live (TTL) values causing outdated cached entries. You can also refer to Microsoft's documentation on non-delivery reports.
Review your system logs for any specific error messages related to DNS queries or network connectivity. These logs can offer valuable insights into why your server is failing to resolve MX records. Look for errors indicating DNS timeouts, unreachable DNS servers, or other network-related issues.
If you're using a specific MTA, such as PowerMTA, investigate if there are any known compatibility issues or recommended configurations for sending to Microsoft domains. For instance, some users have found that adjusting the edns-udp-length setting can resolve certain DNS lookup issues that lead to 5.4.4 errors.
Scenario: Incorrect DNS records
Problem: Your domain's MX or A records are incorrect or missing.
Impact: Recipient servers cannot locate your mail hosts, resulting in 5.4.4 errors.
Scenario: DNS caching issues
Problem: Your server's DNS resolver has cached an outdated MX record.
Impact: Emails fail even if the public DNS is now correct.
Ensuring long-term deliverability
Ensuring a robust DNS infrastructure is fundamental for consistent email deliverability. Use reliable and low-latency DNS resolvers for your sending systems. Also, ensure your firewalls are not inadvertently blocking DNS queries, which can lead to intermittent 5.4.4 errors or broader DNS resolution failures with Microsoft domains.
Proactive monitoring of your email delivery logs and bounce rates is also crucial. A sudden spike in 5.4.4 errors, especially targeting specific domains, is a strong indicator of an issue that requires immediate attention. Early detection allows for quicker diagnosis and resolution, minimizing disruption to your email campaigns. For broader insights, consider how to troubleshoot email deliverability issues.
While not a direct cause of 5.4.4 errors, maintaining strong email authentication (SPF, DKIM, and DMARC) is vital. A good sender reputation can indirectly influence how recipient mail servers process your messages, including DNS queries, and help avoid issues that could lead to your IP or domain being added to a blacklist or blocklist.
Resolving persistent 5.4.4 errors
If you're facing persistent 5.4.4 errors that don't resolve quickly, consider escalating your troubleshooting. For self-managed email servers, meticulously review your network configuration, DNS resolver settings, and any recent changes that might affect outbound connectivity. Sometimes, a simple restart of DNS services can clear cached errors.
For those using third-party sending platforms, check their status pages or contact their support team. They might have internal insights into any temporary network issues or specific configurations needed for Microsoft domains. Remember that a well-configured MTA will have retry mechanisms for transient errors, but a persistent 5.4.4 indicates a deeper problem.
Views from the trenches
Best practices
Ensure your sending infrastructure uses reliable and up-to-date DNS resolvers to minimize lookup failures.
Regularly check DNS propagation for your own domains and critical recipient domains, particularly MX records.
Implement monitoring for DNS resolution performance and email bounce rates to detect issues early.
Common pitfalls
Assuming the issue is always on the recipient's side without first checking your own DNS setup.
Ignoring transient DNS errors, as they can indicate underlying resolver instability.
Not accounting for DNS TTL values, which can delay the propagation of corrected records.
Expert tips
Monitor DNS resolver performance metrics closely to identify any latency or failure trends.
Consider using a redundant set of DNS resolvers for your sending infrastructure to enhance reliability.
Be aware of large email providers' (like Microsoft) temporary network adjustments that can cause brief, widespread issues.
Expert view
Expert from Email Geeks says they noticed the MX record for hotmail.com points to hotmail-com.olc.protection.outlook.com, and if it were a general MX issue, everyone sending to Hotmail would experience it.
2019-09-23 - Email Geeks
Marketer view
Marketer from Email Geeks says they saw the 5.4.4 issue with Hotmail, MSN, and Live domains across the board but that it seemed to resolve itself over a weekend.
2019-09-23 - Email Geeks
Final thoughts on resolving email bounces
The 5.4.4 'no mail hosts' error, while pointing to the recipient domain, is almost always a signal that your sending system is encountering DNS resolution issues when trying to route emails to Microsoft domains.
By systematically checking your DNS resolver, network configuration, and ensuring proper MTA settings, you can effectively diagnose and address these bounces, ensuring your emails reach their intended Microsoft recipients.
Microsoft, this indicates their mail servers could not resolve the necessary DNS information to route your email. While the error originates from the receiving end, the problem is often related to how your system (or its DNS resolver) interacts with their DNS infrastructure.
Outlook.com. Temporary network congestion or DNS server outages affecting the lookup path can also trigger this bounce, often resolving themselves quickly.
Hotmail.com or 
Live.com have valid and resolvable MX entries. Concurrently, check your own mail server's DNS resolver configuration. Make sure it's pointing to reliable DNS servers and that there are no issues with Time To Live (TTL) values causing outdated cached entries. You can also refer to Microsoft's documentation on non-delivery reports.
