diff --git a/src/main.cpp b/src/main.cpp
index 79cc606856a..5949e5285dc 100644
--- a/src/main.cpp
+++ b/src/main.cpp
@@ -811,7 +811,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
     // is it already in the memory pool?
     uint256 hash = tx.GetHash();
     if (pool.exists(hash))
-        return false;
+        return state.Invalid(false, REJECT_ALREADY_KNOWN, "txn-already-in-mempool");
 
     // Check for conflicts with in-memory transactions
     {
@@ -822,7 +822,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
         if (pool.mapNextTx.count(outpoint))
         {
             // Disable replacement feature for now
-            return false;
+            return state.Invalid(false, REJECT_CONFLICT, "txn-mempool-conflict");
         }
     }
     }
@@ -839,7 +839,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
 
         // do we already have it?
         if (view.HaveCoins(hash))
-            return false;
+            return state.Invalid(false, REJECT_ALREADY_KNOWN, "txn-already-known");
 
         // do all inputs exist?
         // Note that this does not check for the presence of actual outputs (see the next check for that),
@@ -848,7 +848,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
             if (!view.HaveCoins(txin.prevout.hash)) {
                 if (pfMissingInputs)
                     *pfMissingInputs = true;
-                return false;
+                return false; // fMissingInputs and !state.IsInvalid() is used to detect this condition, don't set state.Invalid()
             }
         }
 
@@ -868,7 +868,7 @@ bool AcceptToMemoryPool(CTxMemPool& pool, CValidationState &state, const CTransa
 
         // Check for non-standard pay-to-script-hash in inputs
         if (fRequireStandard && !AreInputsStandard(tx, view))
-            return error("AcceptToMemoryPool: nonstandard transaction input");
+            return state.Invalid(false, REJECT_NONSTANDARD, "bad-txns-nonstandard-inputs");
 
         // Check that the transaction doesn't have an excessive number of
         // sigops, making it impossible to mine. Since the coinbase transaction
@@ -1239,7 +1239,7 @@ void static InvalidBlockFound(CBlockIndex *pindex, const CValidationState &state
     if (state.IsInvalid(nDoS)) {
         std::map<uint256, NodeId>::iterator it = mapBlockSource.find(pindex->GetBlockHash());
         if (it != mapBlockSource.end() && State(it->second)) {
-            assert(state.GetRejectCode() < 0x100);
+            assert (state.GetRejectCode() < REJECT_INTERNAL); // Blocks are never rejected with internal reject codes
             CBlockReject reject = {(unsigned char)state.GetRejectCode(), state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), pindex->GetBlockHash()};
             State(it->second)->rejects.push_back(reject);
             if (nDoS > 0)
@@ -4358,8 +4358,9 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
             LogPrint("mempool", "%s from peer=%d %s was not accepted into the memory pool: %s\n", tx.GetHash().ToString(),
                 pfrom->id, pfrom->cleanSubVer,
                 state.GetRejectReason());
-            pfrom->PushMessage("reject", strCommand, state.GetRejectCode(),
-                               state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), inv.hash);
+            if (state.GetRejectCode() < REJECT_INTERNAL) // Never send AcceptToMemoryPool's internal codes over P2P
+                pfrom->PushMessage("reject", strCommand, state.GetRejectCode(),
+                                   state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), inv.hash);
             if (nDoS > 0)
                 Misbehaving(pfrom->GetId(), nDoS);
         }
@@ -4439,6 +4440,7 @@ bool static ProcessMessage(CNode* pfrom, string strCommand, CDataStream& vRecv,
         ProcessNewBlock(state, pfrom, &block, forceProcessing, NULL);
         int nDoS;
         if (state.IsInvalid(nDoS)) {
+            assert (state.GetRejectCode() < REJECT_INTERNAL); // Blocks are never rejected with internal reject codes
             pfrom->PushMessage("reject", strCommand, state.GetRejectCode(),
                                state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), inv.hash);
             if (nDoS > 0) {
diff --git a/src/main.h b/src/main.h
index eec7e6fa595..5c54b11d469 100644
--- a/src/main.h
+++ b/src/main.h
@@ -455,7 +455,16 @@ extern CBlockTreeDB *pblocktree;
  */
 int GetSpendHeight(const CCoinsViewCache& inputs);
 
-/** local "reject" message codes for RPC which can not be triggered by p2p trasactions */
+/** Reject codes greater or equal to this can be returned by AcceptToMemPool
+ * for transactions, to signal internal conditions. They cannot and should not
+ * be sent over the P2P network.
+ */
+static const unsigned int REJECT_INTERNAL = 0x100;
+/** Too high fee. Can not be triggered by P2P transactions */
 static const unsigned int REJECT_HIGHFEE = 0x100;
+/** Transaction is already known (either in mempool or blockchain) */
+static const unsigned int REJECT_ALREADY_KNOWN = 0x101;
+/** Transaction conflicts with a transaction already known */
+static const unsigned int REJECT_CONFLICT = 0x102;
 
 #endif // BITCOIN_MAIN_H