diff --git a/src/SparkPost.Tests/DataMapperTests.cs b/src/SparkPost.Tests/DataMapperTests.cs
index 6d595a72..3a6e54e3 100644
--- a/src/SparkPost.Tests/DataMapperTests.cs
+++ b/src/SparkPost.Tests/DataMapperTests.cs
@@ -259,6 +259,16 @@ public void metadata()
                     .CastAs<IDictionary<string, object>>()[key].ShouldEqual(value);
             }
 
+            [Test]
+            public void do_not_alter_the_keys_passed_to_metadata()
+            {
+                var key = "TEST";
+                var value = Guid.NewGuid().ToString();
+                transmission.Metadata[key] = value;
+                mapper.ToDictionary(transmission)["metadata"]
+                    .CastAs<IDictionary<string, object>>()[key].ShouldEqual(value);
+            }
+
             [Test]
             public void do_not_include_empty_metadata()
             {
diff --git a/src/SparkPost/DataMapper.cs b/src/SparkPost/DataMapper.cs
index ac3ce89b..ba94e6c0 100644
--- a/src/SparkPost/DataMapper.cs
+++ b/src/SparkPost/DataMapper.cs
@@ -84,10 +84,8 @@ public virtual IDictionary<string, object> ToDictionary(Transmission transmissio
         {
             var data = new Dictionary<string, object>
             {
-                ["substitution_data"] =
-                    transmission.SubstitutionData != null && transmission.SubstitutionData.Keys.Any()
-                        ? transmission.SubstitutionData
-                        : null,
+                ["substitution_data"] = AlterThisDictionaryToBePassedToSparkpost(transmission.SubstitutionData),
+                ["metadata"] = AlterThisDictionaryToBePassedToSparkpost(transmission.Metadata),
                 ["recipients"] = transmission.ListId != null
                     ? (object) new Dictionary<string, object> {["list_id"] = transmission.ListId}
                     : transmission.Recipients.Select(ToDictionary)
@@ -100,6 +98,13 @@ public virtual IDictionary<string, object> ToDictionary(Transmission transmissio
             return result;
         }
 
+        private static IDictionary<string, object> AlterThisDictionaryToBePassedToSparkpost(IDictionary<string, object> dictionary)
+        {
+            return dictionary != null && dictionary.Keys.Any()
+                ? dictionary
+                : null;
+        }
+
         public virtual IDictionary<string, object> ToDictionary(Recipient recipient)
         {
             return WithCommonConventions(recipient, new Dictionary<string, object>()