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How to Troubleshoot Outlook 5.4.0 Email Bounce Errors
Question details
Users are experiencing 5.4.0 delivery failure notifications when attempting to send outgoing emails, particularly those containing attachments.
- Product
- Microsoft Outlook
- Device & OS
- not provided
- Scenario
- Sending outgoing emails with attachments to external or internal recipients.
- Observed behavior
- The email fails to deliver and returns a 5.4.0 bounce error due to mail routing issues, size limits, or recipient server restrictions.
Before proceeding, check the total file size of your email attachments and confirm with the recipient if they are currently able to receive external emails.
Verify Message Size Limits and Compress Attachments
Check if your email exceeds the recipient's maximum allowed message size, which is a common trigger for the 5.4.0 delivery failure.
Open the draft of the failed email in Outlook and check the total size of all attached files.
Ensure the combined size is under the standard email limit, which is typically 20MB to 25MB for most mail providers.
If the attachments are too large, compress them into a ZIP file or upload them to a cloud storage service and share the link instead of attaching the files directly.
Remove the original large attachments, insert the cloud link or compressed file, and resend the email to see if it successfully routes.
Review Mail Flow Rules and Connector Settings
For Exchange or hybrid environments, incorrectly configured connectors or mail flow rules can cause routing loops and trigger the 5.4.0 error.
Contact the Recipient's Mail Administrator
If the issue persists, the recipient's server might be rejecting the email due to MX record misconfigurations or aggressive spam filtering.
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Frequently Asked Questions
What does a 5.4.0 email bounce error mean in Outlook?
A 5.4.0 error indicates a routing failure or delivery issue. This means your email server could not find a path to the recipient's server, or the recipient's server actively rejected the message due to policy restrictions like file size limits.
Why do attachments specifically trigger the 5.4.0 delivery failure?
Attachments often trigger this error if they exceed the size limits set by either your email provider or the recipient's server. Additionally, certain file types (like executable files or scripts) might be blocked by the recipient's security filters, causing a bounce.
Can an incorrect MX record cause a 5.4.0 error?
Yes, if the recipient's domain has missing or misconfigured MX (Mail Exchanger) records in their DNS settings, your outgoing email server won't know where to route the message, resulting in a 5.4.0 bounce back.
How do I fix an email routing loop in Microsoft Exchange?
Routing loops typically occur in hybrid setups. An IT administrator must access the Exchange Admin Center, verify the connectors between Exchange Online and on-premises servers, and ensure mail flow rules are not inadvertently bouncing messages back and forth.
