diff --git a/pkg/message/build_rfc822.go b/pkg/message/build_rfc822.go
index 026381f2..fa778006 100644
--- a/pkg/message/build_rfc822.go
+++ b/pkg/message/build_rfc822.go
@@ -494,7 +494,10 @@ func toMessageHeader(hdr mail.Header) message.Header {
 
 	for key, val := range hdr {
 		for _, val := range val {
-			res.Add(key, val)
+			// Using AddRaw instead of Add to save key-value pair as byte buffer within Header.
+			// This buffer is used latter on in message writer to construct message and avoid crash
+			// when key length is more than 76 characters long.
+			res.AddRaw([]byte(key + ": " + val + "\r\n"))
 		}
 	}
 
diff --git a/pkg/message/build_test.go b/pkg/message/build_test.go
index 7278e6db..d95f6824 100644
--- a/pkg/message/build_test.go
+++ b/pkg/message/build_test.go
@@ -52,6 +52,27 @@ func TestBuildPlainMessage(t *testing.T) {
 		expectTransferEncoding(is(`quoted-printable`))
 }
 
+func TestBuildPlainMessageWithLongKey(t *testing.T) {
+	m := gomock.NewController(t)
+	defer m.Finish()
+
+	b := NewBuilder(1, 1, 1)
+	defer b.Done()
+
+	kr := tests.MakeKeyRing(t)
+	msg := newTestMessage(t, kr, "messageID", "addressID", "text/plain", "body", time.Now())
+	msg.Header["ReallyVeryVeryVeryVeryVeryLongLongLongLongLongLongLongKeyThatWillHaveNotSoLongValue"] = []string{"value"}
+
+	res, err := b.NewJob(context.Background(), newTestFetcher(m, kr, msg), msg.ID).GetResult()
+	require.NoError(t, err)
+
+	section(t, res).
+		expectContentType(is(`text/plain`)).
+		expectBody(is(`body`)).
+		expectTransferEncoding(is(`quoted-printable`)).
+		expectHeader(`ReallyVeryVeryVeryVeryVeryLongLongLongLongLongLongLongKeyThatWillHaveNotSoLongValue`, is(`value`))
+}
+
 func TestBuildHTMLMessage(t *testing.T) {
 	m := gomock.NewController(t)
 	defer m.Finish()