diff --git a/encoding.go b/encoding.go
index dff1e6f..0aed3d3 100644
--- a/encoding.go
+++ b/encoding.go
@@ -152,12 +152,17 @@ const (
 	MIMERelated     MIMEType = "related"
 )
 
-// String is a standard method to convert an Encoding into a printable format
-func (e Encoding) String() string {
-	return string(e)
-}
-
 // String is a standard method to convert an Charset into a printable format
 func (c Charset) String() string {
 	return string(c)
 }
+
+// String is a standard method to convert an ContentType into a printable format
+func (c ContentType) String() string {
+	return string(c)
+}
+
+// String is a standard method to convert an Encoding into a printable format
+func (e Encoding) String() string {
+	return string(e)
+}
diff --git a/encoding_test.go b/encoding_test.go
index 71624f3..4a3affc 100644
--- a/encoding_test.go
+++ b/encoding_test.go
@@ -27,6 +27,38 @@ func TestEncoding_String(t *testing.T) {
 	}
 }
 
+// TestContentType_String tests the string method of the ContentType object
+func TestContentType_String(t *testing.T) {
+	tests := []struct {
+		name string
+		ct   ContentType
+		want string
+	}{
+		{"ContentType: text/plain", TypeTextPlain, "text/plain"},
+		{"ContentType: text/html", TypeTextHTML, "text/html"},
+		{
+			"ContentType: application/octet-stream", TypeAppOctetStream,
+			"application/octet-stream",
+		},
+		{
+			"ContentType: application/pgp-signature", TypePGPSignature,
+			"application/pgp-signature",
+		},
+		{
+			"ContentType: application/pgp-encrypted", TypePGPEncrypted,
+			"application/pgp-encrypted",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if tt.ct.String() != tt.want {
+				t.Errorf("wrong string for Content-Type returned. Expected: %s, got: %s",
+					tt.want, tt.ct.String())
+			}
+		})
+	}
+}
+
 // TestCharset_String tests the string method of the Charset object
 func TestCharset_String(t *testing.T) {
 	tests := []struct {
diff --git a/header.go b/header.go
index 4fca337..35c5284 100644
--- a/header.go
+++ b/header.go
@@ -73,6 +73,9 @@ const (
 	// HeaderPriority represents the "Priority" field
 	HeaderPriority Header = "Priority"
 
+	// HeaderReferences is the "References" header field
+	HeaderReferences Header = "References"
+
 	// HeaderReplyTo is the "Reply-To" header field
 	HeaderReplyTo Header = "Reply-To"
 
diff --git a/header_test.go b/header_test.go
index aea2f4c..a060ae6 100644
--- a/header_test.go
+++ b/header_test.go
@@ -87,6 +87,7 @@ func TestHeader_String(t *testing.T) {
 		{"Header: Organization", HeaderOrganization, "Organization"},
 		{"Header: Precedence", HeaderPrecedence, "Precedence"},
 		{"Header: Priority", HeaderPriority, "Priority"},
+		{"Header: HeaderReferences", HeaderReferences, "References"},
 		{"Header: Reply-To", HeaderReplyTo, "Reply-To"},
 		{"Header: Subject", HeaderSubject, "Subject"},
 		{"Header: User-Agent", HeaderUserAgent, "User-Agent"},