What causes a 501 5.5.4 Invalid domain name error when sending emails to Office 365?
Summary
What email marketers say10Marketer opinions
Email marketer from EmailMarketingTips Blog explains that Incorrect SPF or DKIM records can lead to receiving mail servers rejecting the email. Make sure your SPF record includes the sending server's IP address and that DKIM signatures are valid.
Email marketer from Reddit shares that the sending server may not have a proper reverse DNS (PTR) record, or the hostname in the HELO/EHLO greeting doesn't match the IP address. Some receiving mail servers reject connections from servers without proper reverse DNS.
Email marketer from EmailClientGuru Blog explains that a local firewall or antivirus software might be interfering with the SMTP connection. Temporarily disabling these can help determine if they are the cause.
Email marketer from CloudMailPro Knowledge Base explains that the sending mail server might be using an outdated TLS version that is not supported by Office 365. Ensure that the sending server supports TLS 1.2 or later.
Email marketer from Reddit recommends using telnet to manually connect to the mail server and send a test email. This helps identify the exact point at which the error occurs and provides more detailed error messages.
Email marketer from StackExchange explains that often this error means the recipient address has an invalid character. Check for spaces, illegal characters, or common typos. Also, sometimes copying and pasting email addresses can introduce hidden characters.
Email marketer from MailServerPro Forum suggests that the sending server’s IP address might be blacklisted, causing receiving servers to reject the connection. Check your IP against common blacklists using online tools.
Email marketer from Security Blog warns that some receiving mail servers, especially those with aggressive anti-phishing filters, might flag the sending domain as suspicious if it's newly registered or has a poor reputation, leading to a temporary rejection with a 501 error.
Email marketer from SysAdmin Forum suggests that if the email message is very large, it might exceed the Maximum Transmission Unit (MTU) size allowed by the network. Fragmenting the message or reducing its size can resolve the issue.
Email marketer from EmailGurus Forum suggests that a possible cause is that the domain in the recipient address doesn't have a valid MX record in DNS, or the DNS server is temporarily unavailable. Office 365 performs strict validation of domain names.
What the experts say5Expert opinions
Expert from Spam Resource suggests that invalid characters or syntax within the recipient's email address are common causes for the '501 5.5.4 Invalid domain name' error. Ensuring that the address adheres to RFC standards, with no unescaped special characters or spaces, is essential.
Expert from Word to the Wise explains that issues can arise from a domain lacking a valid MX record or from DNS problems that cause lookup failures. Checking DNS settings with tools like `dig` can help diagnose and fix such problems.
Expert from Email Geeks suggests that there might be a domain that has dropped out of DNS, causing some places to refuse emails from domains that don’t accept email back.
Expert from Email Geeks explains that the bounce message received is not from MSN but from something completely different, likely the hotmail/outlook infrastructure.
Expert from Email Geeks suspects the issue is an internal Microsoft problem, potentially with the nameservers for the domain not responding or only being accessible internally due to misconfiguration.
What the documentation says5Technical articles
Documentation from MXToolbox explains that the recipient domain's DNS configuration might be incorrect or incomplete. Specifically, missing or misconfigured MX records can cause this error. Use tools like MXToolbox to diagnose DNS issues.
Documentation from Microsoft Docs explains that a 501 5.5.4 error typically indicates a syntax error in the recipient's email address. This can be due to invalid characters, incorrect formatting, or a typo in the domain name.
Documentation from RFC 5322 details the official specifications for email address syntax, emphasizing that any deviation from allowed characters or formatting rules can lead to a 501 error. Incorrectly escaped characters or misuse of quotes are potential culprits.
Documentation from IETF's RFC 2821 specifies SMTP error codes. The 501 error is defined as 'Syntax error in parameters or arguments' meaning the command sent to the server was syntactically incorrect.
Documentation from Microsoft Docs explains that Microsoft Exchange Online has specific limits on message size, recipient limits, and sending rates. Exceeding these limits can result in temporary rejection with a 501 or similar error code.