What causes a 501 5.5.4 Invalid domain name error when sending emails to Office 365?
Matthew Whittaker
Co-founder & CTO, Suped
Published 22 Jun 2025
Updated 15 Aug 2025
7 min read
When sending emails, encountering a bounce message can be frustrating, especially when it points to an issue that seems unclear. One such error that often arises when communicating with Office 365 recipients is the 501 5.5.4 Invalid domain name bounce.
This error essentially means that the recipient's mail server, in this case, Office 365, is refusing to accept the connection because the domain name presented by your sending server during the initial SMTP handshake is deemed invalid. It is a critical indicator that something is amiss with how your server is identifying itself.
Understanding the 501 5.5.4 error
The 501 SMTP error code generally signifies a syntax error in a command or an invalid command parameter. The additional 5.5.4 specific code points directly to an invalid domain name. It is a permanent failure (meaning the server explicitly refused the connection), indicating that Office 365 did not like the domain name your server used to introduce itself.
During the initial phase of an email transfer, your sending mail server (the client) initiates a conversation with the recipient's mail server (the host) using the HELO or EHLO command. In this command, your server announces its fully qualified domain name (FQDN). If the FQDN provided is malformed, unrecognized, or fails to resolve to a valid IP address, Office 365 will reject the connection with the 501 5.5.4 error.
Office 365, being a large email service provider, maintains stringent checks to combat spam and ensure legitimate email traffic. These checks include validating the sender's identity, which relies heavily on proper DNS configuration and a resolvable FQDN during the SMTP handshake. An invalid domain name is a red flag for their systems.
The primary cause of the 501 5.5.4 error is usually an issue with your sending mail server's configuration, specifically its hostname or associated DNS records. This could mean your server's fully qualified domain name (FQDN) is incorrect, or that its reverse DNS (PTR) record does not correctly resolve. These are foundational elements of email deliverability. For related issues, you might find information on why emails bounce with domain does not exist helpful.
Other less common, but equally impactful, causes include hidden or unusual characters within your server's hostname configuration files (such as /etc/hosts on Linux systems). This kind of subtle corruption can lead to the server presenting a malformed FQDN during the HELO/EHLO handshake. Sometimes, even the SMTP client or library used to send emails can introduce unexpected content, as discussed in a SMTP response question from Microsoft.
While often originating from the sender's setup, there are instances where the issue might stem from the recipient's side, especially with large providers like Office 365, due to internal misconfigurations or temporary issues with their domain resolution. This is less frequent but not impossible, and it's something to consider if your configurations appear flawless.
Sender-side causes
Misconfigured FQDN: The sending server's fully qualified domain name is incorrect or malformed.
Missing rDNS record: The sending IP address lacks a proper reverse DNS (PTR) entry, which many servers require.
Invalid HELO/EHLO: The command sent by the client contains an invalid or unresolvable domain name.
Hidden characters: Unseen spaces or characters in host files can corrupt the domain name.
Recipient-side (Office 365) causes
Internal network issues: Office 365's internal domain resolution or mail routing might have temporary problems.
Temporary DNS resolution: Occasional transient lookup failures can occur within their infrastructure.
Stringent validation: Office 365's strict policies might flag perfectly valid domains under certain edge cases.
Insight from error messages
If the bounce message includes a hostname like HE1EUR01FT042.eop-EUR01.prod.protection.outlook.com, it often suggests that the issue is occurring within Microsoft's internal network or protection systems. While this could be a temporary glitch, it's worth noting the specific domain reported.
Diagnosing the problem
The first step in diagnosing a 501 5.5.4 error is to thoroughly examine the full bounce message you receive. Look for the hostname that Office 365 reports as refusing the connection. This often appears after refused to talk to me: 501 5.5.4 Invalid domain name. Note down any IP addresses or server names mentioned, as these provide clues about where the problem lies.
Next, you need to verify your sending server's hostname and its associated DNS records. Crucially, check your reverse DNS (PTR) record. Many mail servers, including Office 365, perform a reverse DNS lookup on the connecting IP address and expect the PTR record to resolve back to the FQDN presented in the HELO/EHLO command. A mismatch or absence of a PTR record can frequently trigger this invalid domain name error. If you are troubleshooting other related errors, you might want to look into why your emails are going to spam.
Finally, dive into your mail server's logs. These logs capture the entire SMTP conversation between your server and the recipient server, providing a detailed record of the commands sent and responses received. This can help pinpoint if the HELO/EHLO command itself is malformed or if there are any other unexpected parameters that might be disrupting the process. Look for any instances of hidden characters or spaces that could be affecting the domain name.
Check
Description
What to look for
Sending server FQDN
Verify the hostname of your mail server. This is the name your server presents during the HELO/EHLO command.
It should be a valid, fully qualified domain name (e.g., mail.yourdomain.com) and resolvable via DNS.
Reverse DNS (rDNS)
Check if your sending IP address has a proper PTR record. This is a lookup from IP to hostname.
The PTR record should accurately match your server's FQDN. Microsoft Outlook and Office 365 often enforce this.
SMTP logs
Review the detailed SMTP conversation from your mail server's logs.
Identify the exact HELO/EHLO command sent and the immediate 501 5.5.4 response. Look for any anomalies or unexpected characters.
Resolving the 501 5.5.4 error
The most common fix involves ensuring your mail server's hostname is correctly configured and resolvable. Verify that your server's operating system and mail transfer agent (MTA) are set to use a valid Fully Qualified Domain Name (FQDN), such as mail.yourdomain.com, not just yourdomain.com or a generic hostname.
Crucially, you must ensure that your sending IP address has a properly configured reverse DNS (PTR) record that resolves to the same FQDN your server uses in its HELO/EHLO command. This is a fundamental requirement for establishing trust with major mail providers like Office 365. If the PTR record is missing or points to a different name, it will likely result in the 501 5.5.4 error.
If you are sending emails through an application or a third-party library, inspect its configuration to ensure it's not inadvertently adding invalid characters or malforming the domain name sent during the SMTP handshake. Sometimes, even seemingly insignificant details like trailing spaces can cause validation failures.
If, after thorough checks, all your server and DNS configurations appear impeccable, and the error persists specifically with Office 365, it is possible the issue lies within Microsoft's (or the recipient's) infrastructure. In such cases, the problem might resolve itself after some time, or it may require reaching out to their support for further investigation. For other Office 365-related issues, consider checking how to fix SMTP error code 5.4.1.
Best practices for avoiding this error
Validate FQDN: Ensure your sending server's hostname is a fully qualified, publicly resolvable domain name.
Match rDNS: Set up a PTR record for your sending IP that accurately matches your server's FQDN.
Clean configuration: Regularly check host files and SMTP client settings for any hidden characters or extraneous spaces.
Views from the trenches
Best practices
Always ensure your server's HELO/EHLO greeting matches a resolvable FQDN with a corresponding PTR record.
Regularly audit your DNS settings for both forward and reverse lookups, especially after server migrations or IP changes.
Implement robust error logging to capture the full SMTP conversation, which is invaluable for diagnosing obscure errors.
Common pitfalls
Overlooking hidden characters or extra spaces in configuration files, which can invalidate domain names.
Assuming the error is always on your end without thoroughly checking for recipient-side issues, especially with large providers.
Not having a proper reverse DNS record for the sending IP, a common requirement for major email providers.
Expert tips
Verify your 5322.From address is properly formatted and resolvable, as some servers perform checks on it.
If the error persists and your configurations are pristine, consider that it might be an internal issue within the recipient's mail infrastructure, such as Microsoft's.
Always paste the full bounce message and email headers when seeking help, as it provides crucial context for diagnosis.
Marketer view
Marketer from Email Geeks says the bounce message I received didn't seem to be from the intended recipient's domain but rather from an intermediate mail system.
2020-03-19 - Email Geeks
Expert view
Expert from Email Geeks says the 'Delivered to internal network by a host with no rDNS' suggests a reverse DNS issue or an internal network misconfiguration.
2020-03-19 - Email Geeks
Ensuring seamless email delivery
The 501 5.5.4 'Invalid domain name' error, particularly when sending to Office 365, emphasizes the critical role of accurate DNS and server configuration in email deliverability. By diligently ensuring your sending server's FQDN is valid, resolvable, and matches its reverse DNS, you address the most common causes of this issue. While external factors or transient problems with recipient servers can sometimes play a role, maintaining a pristine sending environment is your best defense against such bounces and ensures your messages reach their intended destination.
Microsoft's internal network or protection systems. While this could be a temporary glitch, it's worth noting the specific domain reported.
The PTR record should accurately match your server's FQDN. Microsoft Outlook and Office 365 often enforce this.
