If we don't allow free txs, always send a fee filter

src/net_processing.cpp

@@ -2993,6 +2993,9 @@ bool SendMessages(CNode* pto, CConnman& connman)
         if (pto->nVersion >= FEEFILTER_VERSION && GetBoolArg("-feefilter", DEFAULT_FEEFILTER) &&
             !(pto->fWhitelisted && GetBoolArg("-whitelistforcerelay", DEFAULT_WHITELISTFORCERELAY))) {
             CAmount currentFilter = mempool.GetMinFee(GetArg("-maxmempool", DEFAULT_MAX_MEMPOOL_SIZE) * 1000000).GetFeePerK();
+            // If we don't allow free transactions, then we always have a fee filter of at least minRelayTxFee
+            if (GetArg("-limitfreerelay", DEFAULT_LIMITFREERELAY) <= 0)
+                currentFilter = std::max(currentFilter, ::minRelayTxFee.GetFeePerK());
             int64_t timeNow = GetTimeMicros();
             if (timeNow > pto->nextSendTimeFeeFilter) {
                 static CFeeRate default_feerate(DEFAULT_MIN_RELAY_TX_FEE);