diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 953c197..0e0cf68 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -44,6 +44,7 @@ set(simplemailqt_HEADERS_PRIVATE
 )
 
 if(BUILD_WITH_SMIME)
+    add_definitions(-DWITH_SMIME)
     list(APPEND simplemailqt_SRC
         smime.cpp
     )
diff --git a/src/SimpleMail b/src/SimpleMail
index 0d2526f..efcaf39 100644
--- a/src/SimpleMail
+++ b/src/SimpleMail
@@ -26,3 +26,6 @@
 #include "mimefile.h"
 #include "server.h"
 #include "serverreply.h"
+#ifdef WITH_SMIME
+#include "smime.h"
+#endif