Please review the changes. @slide

Please rebase

Please rebase

I updated my branch. Please review. @slide

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!

Hi 👋

Thanks for the detailed implementation — this looks really well thought out.

While going through the changes, I had a small observation regarding SMTP error parsing. In some cases, servers may return messages where the status code is embedded mid-message (e.g., "Temporary error: 421 Service unavailable").

Would it make sense to extend the parsing logic to handle such patterns as well, or is the current approach sufficient for most providers?

Just sharing this as a thought — great work overall!