Why are we getting DNS resolution failures with outlook.com when sending email?
Matthew Whittaker
Co-founder & CTO, Suped
Published 31 May 2025
Updated 19 Aug 2025
9 min read
Dealing with DNS resolution failures when sending emails to outlook.com can be incredibly frustrating. One moment, emails flow smoothly, and the next, bounces start appearing, citing issues like "no mail hosts for domain" or unexpected DNS timeouts. It feels like an intermittent, unpredictable problem that's hard to pin down. Many of us in the email deliverability space have encountered this, and it often points to a complex interplay of factors, from the recipient's DNS infrastructure to our own mail transfer agent (MTA) configuration. Pinpointing the exact cause requires a deep dive into DNS behavior and how email systems interpret DNS responses.
This challenge highlights the critical role of the Domain Name System (DNS) in email delivery. When you send an email, your mail server needs to query DNS to find the recipient's mail exchange (MX) records. These records tell your server where to send the email. If this lookup fails, or if the response is unexpected, your email won't reach its destination.
The fundamentals of DNS and email flow
DNS is the backbone of the internet, translating human-readable domain names into IP addresses that computers use to identify each other. For email, this translation is crucial. When your mail server attempts to deliver an email, it performs a series of DNS lookups to locate the recipient's mail server. The first step is typically querying for MX records, which specify the mail servers responsible for accepting email for a domain.
A common cause for email delivery issues, including those with Outlook.com, is when these DNS queries either fail or return an incomplete or malformed response. If your server cannot properly resolve the MX record, it will not know where to send the email, resulting in a bounce back to the sender. This can manifest as errors like "DNS query failed" or "no mail hosts for domain."
Interestingly, Microsoft themselves acknowledge that they may not accept email from senders who fail a reverse DNS lookup, as detailed in their Sender Support Troubleshooting guide. This underscores the importance of a properly configured and responsive DNS setup not just for your own sending, but also for recipient servers to correctly identify and accept your mail.
Underlying causes of resolution problems
One of the primary culprits behind intermittent DNS resolution failures with Outlook.com (and other large providers) can be inconsistencies within the authoritative DNS servers themselves. We've observed instances where Microsoft's authoritative DNS might return a small number of records (e.g., 2 MX records) for a given query at one moment, and a significantly larger number (e.g., 30 MX records) for the exact same query just moments later.
This variability in response size can cause problems for certain DNS resolvers and MTAs (Mail Transfer Agents). If a DNS response exceeds the standard UDP packet size limit (512 bytes) without EDNS0 (Extension Mechanisms for DNS 0) support, the response will be truncated. When a response is truncated, the TC (Truncated) bit is set, indicating that the resolver should retry the query using TCP. If the resolver or MTA does not correctly handle this truncated response and retry over TCP, it might incorrectly interpret the lack of an answer section as a legitimate no mail hosts or similar error, leading to email bounces.
Understanding truncated DNS responses
When a DNS response is too large to fit into a single UDP packet, the DNS server may send a truncated response. This is indicated by the "TC" flag in the DNS header. A compliant resolver should then retry the query using TCP to get the full response. If this retry mechanism is not functioning correctly, it can lead to misinterpretations of the DNS lookup results. The impact of such failures can be significant, especially for high-volume senders whose emails might frequently encounter these large responses.
Another specific issue can arise if the local DNS resolver or the MTA's DNS handling code doesn't properly implement EDNS0 or fallback to TCP. Without EDNS0, DNS responses are limited to 512 bytes over UDP. Larger responses necessitate a switch to TCP for the full data. If this mechanism is flawed, even transient inconsistencies in a recipient's authoritative DNS (like Outlook.com's) can lead to email delivery failures, as the sender's system incorrectly concludes that no mail hosts exist.
Diagnosing the core problem
Effective diagnosis of these DNS resolution failures requires a systematic approach. The first step is to consistently monitor your mail server logs for specific error messages related to DNS lookups. Look for phrases like "DNS query failed," "no mail hosts for domain," or "unresolvable hostname." Tracking these errors over time can help identify patterns or specific domains that are more prone to these issues. You can also gain insight into the problem by understanding common non-delivery reports (NDRs) from Microsoft.
When using command-line tools like dig or nslookup, avoid the +short option, as it hides crucial details, especially in the authority section. You need to see the full response to determine if truncation (TC bit set) is occurring or if delegation is being improperly handled. Using tools like dnstrace or online DNS checkers that provide a full trace, like DNSViz, can be invaluable for visualizing the entire DNS resolution path and identifying any anomalies.
Symptoms of DNS resolution failures
Bounce messages: Receiving NDRs with errors like "no mail hosts for domain" or "DNS query failed."
Intermittent delivery: Some emails to the same recipient domain deliver successfully, while others bounce.
Logs show empty answers: Mail server logs indicate DNS lookups return empty answer sections or TTL=0 for legitimate domains.
It's also important to consider if your MTA is caching DNS results. If caching is enabled, an outdated or erroneous cached entry could be contributing to the problem, even if live queries eventually succeed. Consulting your MTA vendor's documentation or support can help you understand its DNS caching behavior and how to clear or configure it for optimal performance.
Resolving DNS truncation issues and improving reliability
If you are consistently seeing truncated responses or "no mail hosts" errors, especially with large domains like outlook.com or hotmail.com, a key solution lies in ensuring your system properly handles large DNS responses. Modern DNS resolvers and operating systems should support EDNS0, which allows for larger UDP packet sizes. If your system is not configured to support EDNS0, or if its UDP buffer is too small, responses exceeding 512 bytes will be truncated.
One potential fix is to adjust the edns-udp-length setting in your MTA's configuration (if applicable, for example, in PowerMTA) or in your system's /etc/resolv.conf file by adding options edns0. This explicitly signals support for larger UDP DNS packets, potentially resolving truncation issues. Setting a value like edns-udp-length 2048 can increase the buffer size and prevent truncation.
Implementing EDNS0 to prevent truncation
To address DNS truncation issues, particularly with large MX record sets returned by providers like outlook.com, ensure your mail server or local resolver supports and utilizes EDNS0. This protocol extends DNS to allow for larger packet sizes over UDP, reducing the likelihood of truncation. If EDNS0 is not properly configured, your system might resort to TCP retries, which some MTAs or network setups might not handle efficiently, leading to delays or outright failures. Adjusting configurations to explicitly set an EDNS0 UDP buffer size, like in PowerMTA, can directly mitigate this problem.
PowerMTA configuration snippettext
edns-udp-length 2048
Even with optimal configurations on your end, remember that Microsoft's DNS can sometimes be inconsistent, providing varying numbers of MX records at different times. This means ongoing monitoring and a resilient MTA that can gracefully handle these fluctuations are essential. If you continue to face challenges, investigate if your specific cloud infrastructure (e.g., AWS) has any packet filters or network configurations that might be interfering with larger DNS responses. Such low-level network issues can be tricky but are critical to address for reliable email deliverability.
Views from the trenches
This DNS resolution problem, particularly when sending to Outlook.com, highlights the often-overlooked complexity of email infrastructure. While issues might appear as straightforward deliverability problems, they frequently trace back to fundamental network protocols like DNS. Successfully navigating these challenges requires a deep understanding of DNS behavior, proper MTA configuration, and diligent monitoring of mail logs. By addressing potential truncation issues and ensuring your systems are equipped to handle varied DNS responses, you can significantly improve your email deliverability rates, especially to major mailbox providers. Learn more about troubleshooting intermittent email delivery failures caused by SPF and DNS issues.
Best practices
Actively monitor mail server logs for DNS-related errors and patterns.
Use comprehensive DNS tracing tools to visualize resolution paths.
Ensure your MTAs and resolvers are configured for EDNS0 support and proper TCP fallback.
Common pitfalls
Relying solely on short DNS queries, which can hide truncation problems.
Assuming all DNS resolvers handle truncated responses identically.
Overlooking network-level packet filters that might interfere with DNS traffic.
Expert tips
Consider setting up a dedicated test zone with large MX records to replicate and diagnose truncation issues in a controlled environment.
If issues persist, capture network traffic using tools like Wireshark to analyze DNS packet behavior between your MTA and resolvers.
Be aware that public DNS resolvers (like Google or Cloudflare) might have query limits that could impact high-volume lookups.
Expert view
Expert from Email Geeks says that simply using '+short' in DNS queries hides critical delegation information in the authority section. One needs to capture the full details, especially with geo-DNS, from the same network where the problem was observed.
2023-08-01 - Email Geeks
Marketer view
Marketer from Email Geeks says that they are seeing intermittent delivery problems to outlook.com with "unable to route: no mail hosts for domain" bounces, even when using public DNS servers like Google and Cloudflare.
2023-08-02 - Email Geeks
By proactively addressing potential DNS misconfigurations and ensuring your systems are robust against common DNS quirks, you can build a more reliable email infrastructure. This proactive approach helps maintain a good sender reputation and improves overall inbox placement for your emails. For further reading, check out what causes common causes of email deliverability problems with Outlook.com.
Enhancing email deliverability through DNS mastery
DNS resolution failures to outlook.com can stem from a variety of sources, from the recipient’s authoritative DNS inconsistencies to how your own mail transfer agent (MTA) or local resolver handles truncated responses. By understanding the underlying DNS mechanisms, thoroughly diagnosing bounce logs, and implementing appropriate configurations like EDNS0 support, you can significantly mitigate these issues. Regular monitoring and staying informed about DNS best practices are crucial for maintaining healthy email deliverability to all major providers.
outlook.com can be incredibly frustrating. One moment, emails flow smoothly, and the next, bounces start appearing, citing issues like "no mail hosts for domain" or unexpected DNS timeouts. It feels like an intermittent, unpredictable problem that's hard to pin down. Many of us in the email deliverability space have encountered this, and it often points to a complex interplay of factors, from the recipient's DNS infrastructure to our own mail transfer agent (MTA) configuration. Pinpointing the exact cause requires a deep dive into DNS behavior and how email systems interpret DNS responses.
Microsoft themselves acknowledge that they may not accept email from senders who fail a reverse DNS lookup, as detailed in their Sender Support Troubleshooting guide. This underscores the importance of a properly configured and responsive DNS setup not just for your own sending, but also for recipient servers to correctly identify and accept your mail.
AWS) has any packet filters or network configurations that might be interfering with larger DNS responses. Such low-level network issues can be tricky but are critical to address for reliable email deliverability.
