send-email: finer-grained SMTP error handling

Code captured errors but did not process them further.
This treated all failures the same without distinguishing SMTP status.

Add handle-smtp_error to extract SMTP status codes using a regex (as
defined in RFC 5321) and handle errors as follows:

- No error present:
	- If a result is provided, return 1 to indicate success.
	- Otherwise, return 0 to indicate failure.

- Error present with a captured three-digit status code:
	- For 4yz (transient errors), return 1 and allow retries.
	- For 5yz (permanent errors), return 0 to indicate failure.
	- For any other recognized status code, return 1, treating it as
	a transient error.

- Error present but no status code found:
	- Return 1 as a transient error.

Signed-off-by: Zheng Yuting <05ZYT30@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-send-email.perl

@@ -1454,14 +1454,40 @@ sub smtp_auth_maybe {
 			$error = $@ || 'Unknown error';
 		};
 
-		# NOTE: SMTP status code handling will be added in a subsequent commit,
-		# return 1 when failed due to non-credential reasons
-		return $error ? 1 : ($result ? 1 : 0);
+		return ($error
+			? handle_smtp_error($error)
+			: ($result ? 1 : 0));
 	});
 
 	return $auth;
 }
 
+sub handle_smtp_error {
+	my ($error) = @_;
+
+	# Parse SMTP status code from error message in:
+	# https://www.rfc-editor.org/rfc/rfc5321.html
+	if ($error =~ /\b(\d{3})\b/) {
+		my $status_code = $1;
+		if ($status_code =~ /^4/) {
+			# 4yz: Transient Negative Completion reply
+			warn "SMTP transient error (status code $status_code): $error";
+			return 1;
+		} elsif ($status_code =~ /^5/) {
+			# 5yz: Permanent Negative Completion reply
+			warn "SMTP permanent error (status code $status_code): $error";
+			return 0;
+		}
+		# If no recognized status code is found, treat as transient error
+		warn "SMTP unknown error: $error. Treating as transient failure.";
+		return 1;
+	}
+
+	# If no status code is found, treat as transient error
+	warn "SMTP generic error: $error";
+	return 1;
+}
+
 sub ssl_verify_params {
 	eval {
 		require IO::Socket::SSL;