diff --git a/eth/handler.go b/eth/handler.go
index 29f224910..7b753ebb4 100644
--- a/eth/handler.go
+++ b/eth/handler.go
@@ -181,13 +181,30 @@ func NewProtocolManager(config *params.ChainConfig, mode downloader.SyncMode, ne
 		return blockchain.CurrentBlock().NumberU64()
 	}
 	inserter := func(blocks types.Blocks) (int, error) {
-		// If fast sync is running, deny importing weird blocks
-		if atomic.LoadUint32(&manager.fastSync) == 1 {
-			log.Warn("Discarded bad propagated block", "number", blocks[0].Number(), "hash", blocks[0].Hash())
+		// If sync hasn't reached the checkpoint yet, deny importing weird blocks.
+		//
+		// Ideally we would also compare the head block's timestamp and similarly reject
+		// the propagated block if the head is too old. Unfortunately there is a corner
+		// case when starting new networks, where the genesis might be ancient (0 unix)
+		// which would prevent full nodes from accepting it.
+		if manager.blockchain.CurrentBlock().NumberU64() < manager.checkpointNumber {
+			log.Warn("Unsynced yet, discarded propagated block", "number", blocks[0].Number(), "hash", blocks[0].Hash())
 			return 0, nil
 		}
-		atomic.StoreUint32(&manager.acceptTxs, 1) // Mark initial sync done on any fetcher import
-		return manager.blockchain.InsertChain(blocks)
+		// If fast sync is running, deny importing weird blocks. This is a problematic
+		// clause when starting up a new network, because fast-syncing miners might not
+		// accept each others' blocks until a restart. Unfortunately we haven't figured
+		// out a way yet where nodes can decide unilaterally whether the network is new
+		// or not. This should be fixed if we figure out a solution.
+		if atomic.LoadUint32(&manager.fastSync) == 1 {
+			log.Warn("Fast syncing, discarded propagated block", "number", blocks[0].Number(), "hash", blocks[0].Hash())
+			return 0, nil
+		}
+		n, err := manager.blockchain.InsertChain(blocks)
+		if err == nil {
+			atomic.StoreUint32(&manager.acceptTxs, 1) // Mark initial sync done on any fetcher import
+		}
+		return n, err
 	}
 	manager.fetcher = fetcher.New(blockchain.GetBlockByHash, validator, manager.BroadcastBlock, heighter, inserter, manager.removePeer)
 
diff --git a/eth/sync.go b/eth/sync.go
index 3e23c5378..e303ef8d4 100644
--- a/eth/sync.go
+++ b/eth/sync.go
@@ -202,8 +202,17 @@ func (pm *ProtocolManager) synchronise(peer *peer) {
 		log.Info("Fast sync complete, auto disabling")
 		atomic.StoreUint32(&pm.fastSync, 0)
 	}
-	atomic.StoreUint32(&pm.acceptTxs, 1) // Mark initial sync done
-	if head := pm.blockchain.CurrentBlock(); head.NumberU64() > 0 {
+	// If we've successfully finished a sync cycle and passed any required checkpoint,
+	// enable accepting transactions from the network.
+	head := pm.blockchain.CurrentBlock()
+	if head.NumberU64() >= pm.checkpointNumber {
+		// Checkpoint passed, sanity check the timestamp to have a fallback mechanism
+		// for non-checkpointed (number = 0) private networks.
+		if head.Time() >= uint64(time.Now().AddDate(0, -1, 0).Unix()) {
+			atomic.StoreUint32(&pm.acceptTxs, 1)
+		}
+	}
+	if head.NumberU64() > 0 {
 		// We've completed a sync cycle, notify all peers of new state. This path is
 		// essential in star-topology networks where a gateway node needs to notify
 		// all its out-of-date peers of the availability of a new block. This failure