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Non-delivery report errors and solutions


Your 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.


The cause for the delivery problem is identified by an NDR code and description.

Normally the delivery problem will be due to user error such as a special mistake in an email address.

However, certain problems are specific to the transfer of email between Microsoft 365 and Signature 365 when using server-side signatures. This article describes these particular problems and solutions.


Specific errors and solutions

Articles documenting specific errors and solutions:

Configuration 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 receive a non-delivery report containing any of the above errors the potential reasons are:

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

Steps to resolve these problems will be added to this article. In the meantime, please contact support for assistance resolving the problem.

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