What does SMTP Bounce Reason 4.1.8 (bad sender's system address) Domain of sender address does not resolve mean?
Michael Ko
Co-founder & CEO, Suped
Published 30 May 2025
Updated 17 Aug 2025
7 min read
Receiving an SMTP bounce message can be perplexing, especially when it points to an issue with your own sending infrastructure that you believe is correctly configured. One such message, 4.1.8 (bad sender's system address) Domain of sender address does not resolve, indicates a problem with the sender's domain or its associated DNS records. It often suggests that the receiving mail server could not look up or verify the domain listed in the sender's email address.
It's important to distinguish between a 4xx SMTP code, which signifies a temporary failure or deferral, and a 5xx code, which is a permanent rejection (or hard bounce). A 4.1.8 error is typically a deferral, meaning the sending server will attempt to re-send the email later. This suggests the issue is often transient or resolvable. Troubleshooting this email bounce error often involves checking your DNS records.
What the 4.1.8 message means
This specific SMTP error code, 4.1.8, tells you that the mail server on the receiving end (the recipient's server) tried to perform a DNS lookup on your sending domain and failed. The phrase bad sender's system address means that the system (domain) you're sending from couldn't be validated through standard DNS queries.
The core issue is usually that the recipient's mail server couldn't find an MX (Mail Exchanger) record or an A (Address) record for your sending domain. These records are crucial for email delivery, as they tell other mail servers where to send replies or verify your domain's existence. Without them, your domain might appear invalid or non-existent to the receiving server. This situation is further explained in resources covering why emails are rejected when sender domains do not exist.
Because it's a 4xx deferral, it suggests that the problem might be temporary. This could be due to a brief network glitch, a temporary DNS server outage on the recipient's side, or simply a slow response from your DNS servers. Your sending system will typically reattempt delivery for a certain period, hoping the DNS issue resolves itself. However, if the underlying DNS problem persists, these deferrals can eventually turn into permanent rejections (hard bounces), impacting your email deliverability.
Here's a breakdown of the DNS records crucial for email deliverability:
DNS record
Purpose for email
Relevance to 4.1.8
MX Record
Directs incoming mail to your mail servers.
Crucial. If missing or incorrect, recipient servers can't find your mail server for reply verification, leading to this error (also seen in no MX bounce reasons).
A Record
Maps a domain name to an IP address.
Needed if no MX record is present (as a fallback) or if the MX record points to a hostname without a proper A record.
SPF Record
Authenticates sending servers, prevents spoofing.
While not directly causing 4.1.8, an invalid or missing SPF record can contribute to overall sender reputation issues, making recipients more wary of your domain.
DKIM Record
Adds a digital signature to emails for integrity and authenticity.
Similar to SPF, a DKIM issue won't directly cause a 4.1.8, but robust authentication builds trust and helps prevent deferrals.
DMARC Record
Specifies how recipient servers should handle emails that fail SPF or DKIM checks.
While not a direct cause, an improperly configured DMARC policy could indirectly contribute to delivery issues if not aligned with your authentication.
Common causes of 4.1.8 errors
The most frequent cause of a 4.1.8 error is an issue with the DNS records for the sending domain. This could range from simple typographical errors in the domain name itself within the email address to more complex issues with the DNS records not being correctly published or propagated. For instance, if your domain lacks an MX or A record, or if these records are misconfigured, recipient servers won't be able to verify your domain, leading to this deferral. We've compiled a separate resource detailing which DNS records are required to solve 450 4.1.8 errors.
Sometimes, the issue isn't with your DNS records directly, but with a temporary problem on the recipient's side. This could involve a momentary DNS resolver glitch, network congestion, or a recipient server being overloaded. Since the 4.1.8 is a transient error, the sending mail server will attempt to re-deliver the email. If the recipient's DNS system or network resolves its temporary 'burp', the email might go through on a subsequent attempt.
Less commonly, some recipient mail servers or anti-spam appliances might apply stricter policies, or their internal DNS caching could be out of sync. While this isn't a direct problem with your domain's DNS, it can manifest as a 4.1.8 if they can't immediately resolve your domain. Additionally, if your sending IP or domain is on an email blacklist or blocklist, even if your DNS is technically correct, some receiving servers might react by delaying or deferring messages as a precursor to blocking them, sometimes with vague DNS-related error messages. To stay on top of such issues, understanding your email error codes is essential.
Diagnosing and troubleshooting
Transient DNS failures
These are temporary issues that typically resolve themselves over time. They might stem from:
Recipient server issues: The receiving mail server had a temporary problem resolving your domain, possibly due to a brief DNS service interruption on their end or a caching issue.
Network congestion: High traffic or network problems between your mail server and the recipient's DNS server could delay or prevent DNS lookups.
The good news is that for transient issues, your mail server will usually retry sending the email, and it often goes through successfully on a later attempt. Monitoring your mail logs will show these retries.
Diagnose sender domain DNS
Start by checking the DNS records for your sending domain, especially the MX (Mail Exchanger) and A (Address) records. These are critical for email systems to resolve your domain. You can use command-line tools like dig or nslookup to do this.
Example DNS record lookupBASH
dig MX yourdomain.com
dig A yourdomain.com
Ensure that your domain has correctly configured SPF and DKIM records as well, as these authenticate your emails and contribute to overall domain reputation, which can indirectly influence how recipient servers handle your mail. Sometimes issues like Yahoo's '451 message temporarily deferred' error can be related to unresolvable sender domains.
A simple but often overlooked step is to double-check the spelling of the sender domain in the bounce message. A typo in the 'From' address domain can easily lead to a 4.1.8 error, as the recipient server attempts to resolve a non-existent domain. This might sound basic, but it's a quick win if it's the culprit.
If your DNS records appear correct and stable, the issue might be temporary on the recipient's side. Monitor your email logs to see if the messages are eventually delivered after several retries. For intermittent issues affecting only a few domains, it's often a transient network or DNS resolver problem on the receiving end, which may resolve without intervention from your side.
Preventing future 4.1.8 issues
Problem: DNS record issues
Missing DNS records: Lack of proper MX or A records for the sending domain.
Incorrect configuration: Typographical errors or outdated information in your DNS zone file.
Propagation delays: Recent DNS changes haven't fully propagated across the internet.
Solution: DNS best practices
Regular audits: Periodically check your domain's DNS records to ensure they are correct and resolve globally. You can use an email deliverability tester.
DNS provider: Use a reliable DNS hosting provider that offers fast propagation and uptime.
Automated checks: Implement monitoring tools to alert you of any DNS resolution issues.
Beyond ensuring proper DNS records, strengthening your email authentication protocols is key to preventing 4.1.8 and other deliverability issues. Implementing and maintaining valid SPF, DKIM, and DMARC records signals to receiving servers that your emails are legitimate and from an authorized source. A proper DMARC record helps ensure that your email policies are enforced across the email ecosystem.
Continuous monitoring of your email deliverability metrics, including bounce rates, deferrals, and blocklist (or blacklist) status, is crucial. If you see a sudden spike in 4.1.8 errors, especially to a small subset of domains, it might indicate a specific issue with those recipient servers or a temporary DNS problem impacting them. Regular blocklist checks can help identify if your sending IP or domain has been inadvertently listed, which could indirectly contribute to such errors if recipient servers become overly cautious.
Maintaining a strong sender reputation is also vital. This involves consistently sending valuable, wanted emails to engaged recipients, avoiding spam traps, and promptly removing invalid email addresses from your lists. A poor reputation can make recipient servers more scrutinizing of your emails, potentially leading to deferrals or blocks even if your DNS is technically sound.
Views from the trenches
Best practices
Maintain perfectly configured MX and A records for all sending domains.
Regularly monitor your DNS records for proper propagation and resolution globally.
Implement and correctly configure SPF, DKIM, and DMARC for robust email authentication.
Ensure your email lists are clean and free of invalid or unengaged addresses to protect sender reputation.
Investigate persistent 4.1.8 errors with the recipient domain's administrators if possible.
Common pitfalls
Overlooking simple typos in the sender's domain or email address causing resolution failures.
Assuming that a 4xx code is always a hard bounce and immediately removing recipients.
Neglecting to monitor DNS changes or trusting that they will propagate instantly.
Not maintaining strong email authentication, leading to recipient server distrust.
Ignoring small volumes of 4.1.8 errors, which could escalate into larger deliverability problems.
Expert tips
The first step is always to verify your domain's DNS. Use public DNS lookup tools to ensure your MX and A records are correctly published and accessible from different network locations.
Remember that 4xx errors are deferrals, not permanent bounces. Your system will retry. Only escalate if the issue persists after multiple retries or for a significant period.
If only a few specific domains are returning 4.1.8, the issue might be on their end (e.g., temporary DNS resolver problems). It's not always a reflection of your setup.
Ensure your DNS TTL (Time To Live) values are reasonable. Very high TTLs can delay the propagation of necessary updates.
Consider engaging with the recipient's postmaster if the issue is persistent and isolated, as they might provide specific insight into their server's policy or temporary network issues.
Marketer view
Marketer from Email Geeks says they had a client successfully sending emails for years, but suddenly started receiving 4.1.8 errors for a small number of domains. They were trying to understand what might have changed with their DNS configuration to cause this.
2023-06-20 - Email Geeks
Expert view
Expert from Email Geeks says that sometimes a temporary DNS issue on the receiving side can cause these deferrals. They suggested checking if the target domains share a common DNS infrastructure that might have experienced a transient 'burp'.
2023-06-20 - Email Geeks
Final thoughts
The SMTP bounce reason 4.1.8 (bad sender's system address) Domain of sender address does not resolve is a deferral indicating that the recipient's mail server could not resolve your sending domain's DNS records, such as MX or A. While often temporary, it's crucial to identify the root cause, whether it's an issue with your DNS configuration, a transient network problem, or an overly strict recipient server.
By diligently verifying your DNS records, ensuring accurate authentication (SPF, DKIM, DMARC), and consistently monitoring your email sending performance, you can significantly reduce the occurrence of these errors and ensure your messages reach their intended inboxes without unnecessary delays or blocks.
High traffic or network problems between your mail server and the recipient's DNS server could delay or prevent DNS lookups.
