diff --git a/src/classes/Smtpapi.cls b/src/classes/Smtpapi.cls
index a041fe9..324ca72 100644
--- a/src/classes/Smtpapi.cls
+++ b/src/classes/Smtpapi.cls
@@ -57,10 +57,7 @@ global class Smtpapi {
     }
 
     public Header addSubstitution(String key, List<String> val) {
-      for (Integer i = 0; i < val.size(); i++) {
-        List<String> val_in_array = new List<String> { val[i] };
-        this.sub.put(key, val_in_array);
-      }
+      this.sub.put(key, val);
       return this;
     }
 
@@ -117,4 +114,4 @@ global class Smtpapi {
       return this;
     }
   }
-}
\ No newline at end of file
+}