Issue #11629 has been reported by Robert Pankowecki.

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Feature #11629: Implement Enhanced Mail System Status Codes (rfc1893)
https://bugs.ruby-lang.org/issues/11629

* Author: Robert Pankowecki
* Status: Open
* Priority: Normal
* Assignee: 
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Here is a list of exceptions raised by `net/smtp.rb`

~~~
      def exception_class
        case @status
        when /\A4/  then SMTPServerBusy
        when /\A50/ then SMTPSyntaxError
        when /\A53/ then SMTPAuthenticationError
        when /\A5/  then SMTPFatalError
        else             SMTPUnknownError
        end
      end
~~~

When the server responds with `Net::SMTPServerBusy: 401 4.1.3 Bad recipient address syntax` it raises `SMTPServerBusy` where it could use the enhanced information and throw a more specific error such as `BadMailboxAddressSyntax` based on what the RFC says:

~~~
       X.1.3   Bad destination mailbox address syntax

          The destination address was syntactically invalid.  This can
          apply to any field in the address.  This code is only useful
          for permanent failures.
~~~

Then it would be easier for the client to catch such specific exceptions and handle them properly instead of dealing with more general ones.



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