diff --git a/dummy-chain-cert.pem.license b/dummy-chain-cert.pem.license
new file mode 100644
index 0000000..615ea2b
--- /dev/null
+++ b/dummy-chain-cert.pem.license
@@ -0,0 +1,3 @@
+// SPDX-FileCopyrightText: Copyright (c) 2022-2024 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
diff --git a/dummy-chain-key.pem.license b/dummy-chain-key.pem.license
new file mode 100644
index 0000000..615ea2b
--- /dev/null
+++ b/dummy-chain-key.pem.license
@@ -0,0 +1,3 @@
+// SPDX-FileCopyrightText: Copyright (c) 2022-2024 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
diff --git a/smime.go b/smime.go
index 37fb536..62e07aa 100644
--- a/smime.go
+++ b/smime.go
@@ -1,3 +1,7 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
 package mail
 
 import (
diff --git a/smime_test.go b/smime_test.go
index bf4e2d0..9c5b121 100644
--- a/smime_test.go
+++ b/smime_test.go
@@ -1,3 +1,7 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
 package mail
 
 import (
diff --git a/util_test.go b/util_test.go
index 33615d8..f14f010 100644
--- a/util_test.go
+++ b/util_test.go
@@ -1,3 +1,7 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
 package mail
 
 import (