[bug] Wrong exit status.

src/smtpc/smtpc.c

@@ -40,10 +40,10 @@
 
 #define SMTPC_BUFSIZ (8192)
 
-#define SMTPC_ERR_SOCKET (-1)
-#define SMTPC_ERR_PROTOCOL (-2)
-#define SMTPC_ERR_SUBMISSION (-3)
-#define SMTPC_ERR_UNKNOWN (-4)
+#define SMTPC_ERR_SOCKET (255)
+#define SMTPC_ERR_PROTOCOL (254)
+#define SMTPC_ERR_SUBMISSION (253)
+#define SMTPC_ERR_UNKNOWN (252)
 
 #define SMTPC_7BIT (0)
 #define SMTPC_8BIT (1)