diff --git a/src/filter/autotests/gpghome/gpg.conf b/src/filter/autotests/gpghome/gpg.conf
new file mode 100644
index 0000000..228993a
--- /dev/null
+++ b/src/filter/autotests/gpghome/gpg.conf
@@ -0,0 +1,3 @@
+# TODO: replace test keys with some which do not expire
+# so this special setup is not needed and the test can check normal operation
+faked-system-time 20180110T154812
diff --git a/src/util/cryptoutils.cpp b/src/util/cryptoutils.cpp
index e8169a7..9ee39f5 100644
--- a/src/util/cryptoutils.cpp
+++ b/src/util/cryptoutils.cpp
@@ -166,9 +166,17 @@ KMime::Message::Ptr CryptoUtils::assembleMessage(const KMime::Message::Ptr &orig
     out->setBody(const_cast<KMime::Content *>(newContent)->encodedBody());
     out->parse();
 
+    // remove default explicit content headers added by KMime::Content::parse()
+    QVector<KMime::Headers::Base *> headers = out->headers();
+    for (const auto hdr : std::as_const(headers)) {
+        if (isContentHeader(hdr)) {
+            out->removeHeader(hdr->type());
+        }
+    }
+
     // Copy over headers from the original message, except for CT, CTE and CD
     // headers, we want to preserve those from the new content
-    QVector<KMime::Headers::Base *> headers = orig->headers();
+    headers = orig->headers();
     for (const auto hdr : std::as_const(headers)) {
         if (isContentHeader(hdr)) {
             continue;