Add ErrorCode method to SendError

Implemented ErrorCode method to retrieve the error code from the server response in SendError. This method distinguishes between server-generated errors and client-generated errors, returning 0 for errors generated by the client.

senderror.go

@@ -200,6 +200,21 @@ func (e *SendError) EnhancedStatusCode() string {
 	return e.enhancedStatusCode
 }
 
+// ErrorCode returns the error code of the server response.
+//
+// This function retrieves the error code the error returned by the server. The error code will
+// start with 5 on permanent errors and with 4 on a temporary error. If the error is not returned
+// by the server, but is generated by go-mail, the code will be 0.
+//
+// Returns:
+//   - The error code as returned by the server, or 0 if not a server error.
+func (e *SendError) ErrorCode() int {
+	if e == nil {
+		return 0
+	}
+	return e.errcode
+}
+
 // String satisfies the fmt.Stringer interface for the SendErrReason type.
 //
 // This function converts the SendErrReason into a human-readable string representation based