Most common causes
- Application code generated more than one mailbox in From
- A library combined author identities without adding Sender
- Template or MIME construction produced an invalid originator header set
What to verify next
- Inspect the raw From and Sender headers
- Change the application to emit one From address when multiple authors are unnecessary
- If multiple From addresses are intentional, provide the required Sender identity
- Retest the generated raw message rather than only the rendered content
Best diagnostic path
Live analysis
Mail Failure Doctor
Classify the complete rejection and preserve provider-specific diagnostic context.
Open analysis → Live analysisEmail Header Forensics
Inspect From, Sender, Reply-To, Return-Path and transport evidence in the raw message.
Open analysis → Live analysisEmail Infrastructure Digital Twin
Map the domain’s public mail infrastructure and provider relationships before remediation.
Open analysis →Why the exact message matters
The same status family can be triggered by different conditions, and providers frequently add diagnostic text that narrows the issue. Use the complete rejection text rather than treating the numeric code as a complete diagnosis.
Provider reference
For the provider-defined meaning and current requirements, review Microsoft Exchange Online NDR reference.