Add new Outlook/Microsoft SMTP error codes from production data

[juenology]

Add New Outlook/Microsoft SMTP Error Codes

Summary

This PR adds 7 new SMTP error codes for Microsoft/Outlook that were discovered through analysis of production email traffic. These codes are not currently documented in the SMTP Field Manual but are actively being returned by Microsoft's email infrastructure.

Data Source

The error codes were aggregated from 7,484 Microsoft/Outlook SMTP deferrals observed in production marketing email traffic during October 2025. The data has been sanitized following the contribution guidelines (no personal information, no unique server identifiers).

New Error Codes Added

450 (Temporary Failures)

1. 450 4.7.0 - Message not proxied

   • Occurrences: 2
   • Severity: Low (1)
   • Temporary error when message cannot be proxied through Microsoft's infrastructure

2. 450 4.7.1 - Unable to Relay. ATTR14

   • Occurrences: 1
   • Severity: Medium (2)
   • Temporary relay failure

451 (Temporary Failures)

3. 451 4.4.3 - Temporary server error (ATTR18)

   • Occurrences: 20
   • Severity: Low (0)
   • Similar to existing ATTR17 error but with ATTR18 attribute

4. 451 4.4.22 - Message failed to be replicated: no healthy peers found

   • Occurrences: 1
   • Severity: Low (1)
   • More specific variant of the existing 4.4.0 replication error

5. 451 4.7.700 - PFA agent busy

   • Occurrences: 2
   • Severity: Low (1)
   • Protocol Filter Agent busy, requires retry

451 (Updated Description)

6. 451 4.7.0 - Temporary server error (PRX2/PRX3/PRX10 variations)
   • Occurrences: 847 (11.3% of dataset!)
   • Severity: Low (1)
   • Updated the existing entry to clarify this error includes multiple proxy infrastructure variations (PRX2, PRX3, PRX10, etc.)

550 (Permanent Failures) - CRITICAL

7. 550 5.7.515 - Access denied, authentication level not met ⚠️
   • Occurrences: 6,583 (88% of dataset!)
   • Severity: High (4)
   • This is the most significant addition: Microsoft's new strict email authentication enforcement (2024-2025)
   • Indicates SPF, DKIM, or DMARC authentication failure
   • This is a permanent error that requires fixing authentication configuration

Why These Codes Matter

High Impact Errors

The two most frequently occurring errors (451 4.7.0 and 550 5.7.515) together represent 99.3% of all Microsoft deferrals in our dataset:

• 550 5.7.515: 88% of errors - Authentication enforcement
• 451 4.7.0: 11.3% of errors - Proxy infrastructure issues

Microsoft's Authentication Enforcement

The 550 5.7.515 error is particularly important as it represents Microsoft's relatively recent (2024-2025) stricter enforcement of email authentication standards. Senders seeing this error must ensure their:

• SPF records are properly configured
• DKIM signing is working correctly
• DMARC policies are in place

This error will become increasingly common as Microsoft continues to enforce these requirements.

Testing

• ✅ All JSON files validate against the existing schema
• ✅ Error codes follow the established format (smtp;CODE STATUS Message)
• ✅ Severity levels are consistent with existing patterns
• ✅ Personal information and server-specific identifiers have been sanitized
• ✅ Links point to official Microsoft documentation where available

Documentation Followed

This PR follows the contribution guidelines at https://smtpfieldmanual.com/contribute:

• No personal information (emails, names) in responses
• No unique mail server names or identifiers (replaced with generic forms)
• Focused on major email service provider (Microsoft/Outlook)
• Proper JSON formatting maintained

Impact

This contribution will help email developers and administrators:

1. Better understand Microsoft's evolving SMTP error responses
2. Quickly diagnose authentication-related delivery failures (5.7.515)
3. Recognize temporary proxy infrastructure errors (4.7.0 variants)
4. Implement proper retry logic for temporary failures

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Data Analysis Source: Analysis of 7,497 production SMTP deferrals from October 2025
Contributor: Mary-Juen Sohn (@juenology)

[fm]

This project seems dead as the maintainers seem to have left the company. I've since created a vendor-neutral replacement, if you'd like to add your contributions there? -> https://github.com/fm/smtp-codes