diff --git a/.env.example b/.env.example
index f2df123..5616157 100644
--- a/.env.example
+++ b/.env.example
@@ -2,4 +2,6 @@ NET_ID=42
 RPC_URL=https://kovan.infura.io/v3/a3f4d001c1fc4a359ea70dd27fd9cb51
 PRIVATE_KEY=
 ETH_MIXER_ADDRESS=0x1Cea940cA15a303A0E01B7F8589F39fF34308DB2
-DAI_MIXER_ADDRESS=0x7ed3fC8042e18db889A0466F49c438bB1410b3c7
\ No newline at end of file
+DAI_MIXER_ADDRESS=0x7ed3fC8042e18db889A0466F49c438bB1410b3c7
+
+APP_PORT=8000
\ No newline at end of file
diff --git a/config.js b/config.js
index 4cc7a4f..7aa7bbe 100644
--- a/config.js
+++ b/config.js
@@ -14,5 +14,6 @@ module.exports = {
   } ],
   defaultGasPrice: 2,
   gasOracleUrls: ['https://www.etherchain.org/api/gasPriceOracle', 'https://gasprice.poa.network/'],
-  ethdaiAddress: '0x7Ef645705cb7D401C5CD91a395cfcc3Db3C93689'
+  ethdaiAddress: '0x7Ef645705cb7D401C5CD91a395cfcc3Db3C93689',
+  port: process.env.APP_PORT
 }
\ No newline at end of file
diff --git a/index.js b/index.js
index 8851e04..106a21e 100644
--- a/index.js
+++ b/index.js
@@ -1,6 +1,6 @@
 const { numberToHex, toWei, toHex, toBN, toChecksumAddress } = require('web3-utils')
-const { netId, rpcUrl, privateKey, mixers, defaultGasPrice } = require('./config')
-const { fetchGasPrice, isValidProof, isValidArgs, fetchDAIprice, isKnownContract } = require('./utils')
+const { netId, rpcUrl, privateKey, mixers, defaultGasPrice, port } = require('./config')
+const { fetchGasPrice, isValidProof, isValidArgs, fetchDAIprice, isKnownContract, isEnoughFee } = require('./utils')
 const Web3 = require('web3')
 const express = require('express')
 
@@ -70,32 +70,15 @@ app.post('/relay', async (req, resp) => {
     toBN(args[5])
   ]
 
+  if (currency === 'eth' && !refund.isZero()) {
+    return resp.status(400).json({ error: 'Cannot send refund for eth currency.' })
+  }
+
   if (relayer !== web3.eth.defaultAccount) {
     console.log('This proof is for different relayer:', relayer)
     return resp.status(400).json({ error: 'Relayer address is invalid' })
   }
 
-  const expense = toBN(toWei(gasPrices.fast.toString(), 'gwei')).mul(toBN('1000000'))
-  let desiredFee
-  switch (currency) {
-    case 'eth': {
-      if (!refund.isZero()) {
-        return resp.status(400).json({ error: 'Cannot send refund for eth currency.' })
-      }
-      desiredFee = expense
-      break
-    }
-    case 'dai': {
-      desiredFee = expense.add(refund).mul(toBN(ethPriceInDai)).div(toBN(10 ** 18))
-      break
-    }
-  }
-
-  if (fee.lt(desiredFee)) {
-    console.log('Fee is too low')
-    return resp.status(400).json({ error: 'Fee is too low. Try to resend.' })
-  }
-
   try {
     const mixer = new web3.eth.Contract(mixerABI, req.body.contract)
     const isSpent = await mixer.methods.isSpent(nullifierHash).call()
@@ -106,6 +89,7 @@ app.post('/relay', async (req, resp) => {
     if (!isKnownRoot) {
       return resp.status(400).json({ error: 'The merkle root is too old or invalid.' })
     }
+
     const withdrawArgs = [
       proof,
       root,
@@ -115,14 +99,23 @@ app.post('/relay', async (req, resp) => {
       fee.toString(),
       refund.toString()
     ]
-    const gas = await mixer.methods.withdraw(...withdrawArgs).estimateGas({ 
+    let gas = await mixer.methods.withdraw(...withdrawArgs).estimateGas({ 
       from: web3.eth.defaultAccount,
       value: refund 
     })
+
+    gas += 50000
+
+    const { isEnough, reason } = isEnoughFee({ gas, gasPrices, currency, refund, ethPriceInDai, fee })
+    if (!isEnough) {
+      console.log(`Wrong fee: ${reason}`)
+      return resp.status(400).json({ error: reason })
+    }
+
     const result = mixer.methods.withdraw(...withdrawArgs).send({
       from: web3.eth.defaultAccount,
       value: refund,
-      gas: numberToHex(gas + 50000),
+      gas: numberToHex(gas),
       gasPrice: toHex(toWei(gasPrices.fast.toString(), 'gwei')),
       // TODO: nonce
     })
@@ -139,7 +132,7 @@ app.post('/relay', async (req, resp) => {
   }
 })
 
-app.listen(8000)
+app.listen(port || 8000)
 
 if (Number(netId) === 1) {
   fetchGasPrice({ gasPrices })
@@ -147,7 +140,7 @@ if (Number(netId) === 1) {
   console.log('Gas price oracle started.')
 }
 
-console.log('Relayer started')
+console.log('Relayer started on port', port || 8000)
 console.log(`relayerAddress: ${web3.eth.defaultAccount}`)
 console.log(`mixers: ${JSON.stringify(mixers)}`)
 console.log(`gasPrices: ${JSON.stringify(gasPrices)}`)
diff --git a/utils.js b/utils.js
index bf1b9fb..c2510ec 100644
--- a/utils.js
+++ b/utils.js
@@ -1,5 +1,5 @@
 const fetch = require('node-fetch')
-const { isHexStrict, hexToNumberString } = require('web3-utils')
+const { isHexStrict, hexToNumberString, toBN, toWei } = require('web3-utils')
 const { gasOracleUrls, ethdaiAddress, mixers } = require('./config')
 const oracleABI = require('./abis/ETHDAIOracle.json')
 
@@ -97,4 +97,23 @@ function sleep(ms) {
   return new Promise(resolve => setTimeout(resolve, ms))
 }
 
-module.exports = { fetchGasPrice, isValidProof, isValidArgs, sleep, fetchDAIprice, isKnownContract }
+function isEnoughFee({ gas, gasPrices, currency, refund, ethPriceInDai, fee }) {
+  const expense = toBN(toWei(gasPrices.fast.toString(), 'gwei')).mul(toBN(gas))
+  let desiredFee
+  switch (currency) {
+    case 'eth': {
+      desiredFee = expense
+      break
+    }
+    case 'dai': {
+      desiredFee = expense.add(refund).mul(toBN(ethPriceInDai)).div(toBN(10 ** 18))
+      break
+    }
+  }
+  if (fee.lt(desiredFee)) {
+    return { isEnough: false, reason: 'Not enough fee' }
+  } 
+  return { isEnough: true }
+}
+
+module.exports = { fetchGasPrice, isValidProof, isValidArgs, sleep, fetchDAIprice, isKnownContract, isEnoughFee }