diff --git a/encoding.go b/encoding.go
index 77243fc..dbe57ee 100644
--- a/encoding.go
+++ b/encoding.go
@@ -157,7 +157,7 @@ const (
 	MIMEAlternative MIMEType = "alternative"
 	MIMEMixed       MIMEType = "mixed"
 	MIMERelated     MIMEType = "related"
-	MIMESMime       MIMEType = `signed; protocol="application/pkcs7-signature"; micalg=sha256`
+	MIMESMime       MIMEType = `signed; protocol="application/pkcs7-signature"; micalg=sha-256`
 )
 
 // String is a standard method to convert an Charset into a printable format
diff --git a/msgwriter_test.go b/msgwriter_test.go
index ca1a982..3466199 100644
--- a/msgwriter_test.go
+++ b/msgwriter_test.go
@@ -173,7 +173,7 @@ func TestMsgWriter_writeMsg_SMime(t *testing.T) {
 	mw := &msgWriter{writer: &buf, charset: CharsetUTF8, encoder: mime.QEncoding}
 	mw.writeMsg(m)
 	ms := buf.String()
-	if !strings.Contains(ms, `multipart/signed; protocol="application/pkcs7-signature"; micalg=sha256;`) {
+	if !strings.Contains(ms, `multipart/signed; protocol="application/pkcs7-signature"; micalg=sha-256;`) {
 		t.Errorf("writeMsg failed. Expected PGP encoding header but didn't find it in message output")
 	}
 }