[JENKINS-68518] Email-ext treats transient SMTP errors as fatal errors

[akash-manna-sky]

[JENKINS-68518] Email-ext treats transient SMTP errors as fatal errors

Fixes #1380

Testing done

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[akash-manna-sky]

Please review the changes. @slide

[slide]

See inline comment

[slide]

Please rebase

[akash-manna-sky]

Please rebase

I updated my branch. Please review. @slide

[ArpanC6]

Hi @akash-manna-sky! The implementation looks solid. A few observations after reviewing the code:
Strengths:

isTransientSmtpError() correctly handles exception chaining via getNextException() and getCause()
Reflection-based getSmtpReturnCode() is a smart approach to avoid compile-time dependency on SMTP classes
Test coverage is comprehensive — boundary cases (399, 400, 499, 500) are well covered

One suggestion: In parseSmtpErrorCode(), the regex ^(\d{3})\s only matches codes at the start of the message. Some SMTP servers return messages like "Temporary error: 421 Service unavailable" where the code is not at the start. Consider also checking mid-message patterns.
All 17 CI checks are passing and the rebase looks clean. Great work!