Small improvement

app/src/main/java/eu/faircode/email/Core.java

@@ -1249,7 +1249,7 @@ class Core {
                             Date received = isub[j].getReceivedDate();
                             boolean unseen = (sync_unseen && !isub[j].isSet(Flags.Flag.SEEN));
                             boolean flagged = (sync_flagged && isub[j].isSet(Flags.Flag.FLAGGED));
-                            if (received != null && received.getTime() < sync_time && !unseen && !flagged) {
+                            if (received != null && received.getTime() < keep_time && !unseen && !flagged) {
                                 long uid = ifolder.getUID(isub[j]);
                                 Log.i(folder.name + " Skipping old uid=" + uid + " date=" + received);
                                 ids[from + j] = null;