Non-delivery report errors and solutions

If an email processed by Signature 365 server-side could not be delivered and you have received a non-delivery report from Signature 365 or Microsoft 365, use the NDR code and description to identify the cause. If you need a refresher on the routing model, see How Microsoft 365 server-side signatures work.

Delivery problems are often due to user error, such as a mistake in a recipient email address. This article focuses on the issues that are specific to the transfer of email between Microsoft 365 and Signature 365 when using server-side signatures.

Specific errors and solutions

Articles documenting specific errors and solutions:

• Non-delivery report 550 5.1.8 Access denied, bad outbound sender
• Non-delivery report 550 5.4.1 Recipient address rejected: Access denied
• Non-delivery report 552 5.3.4 Header exceeds fixed maximum size
• Non-delivery report 550 5.7.1 Unable to relay this email
• Non-delivery report 550 5.1.8 UTF-8 addresses not supported

Configuration-related issues

Errors that only occur in relation to configuration issues:

• 550 5.7.64 Relay Access Denied ATTR36
• 451 4.4.5 Address domain different from previous accepted address. All recipients must be on the same domain ATTR46
• 451 4.4.62 Mail sent to the wrong Office 365 region. ATTR35
• 454 4.7.0 Failed to establish appropriate TLS channel ATTR41: CertificateExpired: Access Denied

If you receive one of these errors, the most common causes are:

1. The Signature 365 inbound and or outbound connectors are incorrectly configured in your Microsoft 365 tenant.
2. The Signature 365 certificate domain is missing or incorrect in your Microsoft 365 tenant.
3. The Signature 365 certificate for your tenant is missing, invalid, or expired.

Review the following articles to resolve configuration-related NDRs:

• Recreating the Signature 365 Exchange connectors
• Enabling server-side injection in Signature 365
• How to update the Sender Policy Framework (SPF) record
• Signature 365 ports and URLs

Next steps

If you still need help after checking the linked articles, contact support with the NDR code and a copy of the relevant message headers.