[DOCKER][MAIL] Improved exec.sh

docker/mail/exec.sh

@@ -1,4 +1,11 @@
 #!/bin/sh
+USAGE = "Usage: $0 new domain <domain_name>\n     : $0 new user <user_name> <user_password>\n     : $0 new alias <user_alias> <user_name>"
+
+if [ -z "$2" ]
+then
+  echo "$USAGE";
+  exit 1;
+fi
 
 if [ "$1" == "new" ]
 then