diff --git a/core/src/main/java/ch/dissem/bitmessage/BitmessageContext.java b/core/src/main/java/ch/dissem/bitmessage/BitmessageContext.java
index 23e8eff..2307958 100644
--- a/core/src/main/java/ch/dissem/bitmessage/BitmessageContext.java
+++ b/core/src/main/java/ch/dissem/bitmessage/BitmessageContext.java
@@ -31,8 +31,6 @@ import org.slf4j.LoggerFactory;
 
 import java.net.InetAddress;
 import java.util.List;
-import java.util.Timer;
-import java.util.TimerTask;
 import java.util.concurrent.CancellationException;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.Future;
@@ -71,16 +69,10 @@ public class BitmessageContext {
     private BitmessageContext(Builder builder) {
         ctx = new InternalContext(builder);
         labeler = builder.labeler;
+        ctx.getProofOfWorkService().doMissingProofOfWork();
+
         networkListener = new DefaultMessageListener(ctx, labeler, builder.listener);
-
         sendPubkeyOnIdentityCreation = builder.sendPubkeyOnIdentityCreation;
-
-        new Timer().schedule(new TimerTask() {
-            @Override
-            public void run() {
-                ctx.getProofOfWorkService().doMissingProofOfWork();
-            }
-        }, 30_000); // After 30 seconds
     }
 
     public AddressRepository addresses() {
diff --git a/core/src/main/java/ch/dissem/bitmessage/DefaultMessageListener.java b/core/src/main/java/ch/dissem/bitmessage/DefaultMessageListener.java
index eb27dd9..05961e4 100644
--- a/core/src/main/java/ch/dissem/bitmessage/DefaultMessageListener.java
+++ b/core/src/main/java/ch/dissem/bitmessage/DefaultMessageListener.java
@@ -48,9 +48,12 @@ class DefaultMessageListener implements NetworkHandler.MessageListener {
     }
 
     @Override
+    @SuppressWarnings("ConstantConditions")
     public void receive(ObjectMessage object) throws IOException {
         ObjectPayload payload = object.getPayload();
-        if (payload.getType() == null) return;
+        if (payload.getType() == null && payload instanceof GenericPayload) {
+            receive((GenericPayload) payload);
+        }
 
         switch (payload.getType()) {
             case GET_PUBKEY: {
@@ -125,7 +128,7 @@ class DefaultMessageListener implements NetworkHandler.MessageListener {
                 if (!object.isSignatureValid(plaintext.getFrom().getPubkey())) {
                     LOG.warn("Msg with IV " + object.getInventoryVector() + " was successfully decrypted, but signature check failed. Ignoring.");
                 } else {
-                    receive(object.getInventoryVector(), msg.getPlaintext());
+                    receive(object.getInventoryVector(), plaintext);
                 }
                 break;
             } catch (DecryptionFailedException ignore) {
@@ -133,6 +136,16 @@ class DefaultMessageListener implements NetworkHandler.MessageListener {
         }
     }
 
+    protected void receive(GenericPayload ack) {
+        if (ack.getData().length == Msg.ACK_LENGTH) {
+            Plaintext msg = ctx.getMessageRepository().getMessageForAck(ack.getData());
+            if (msg != null) {
+                ctx.getLabeler().markAsAcknowledged(msg);
+                ctx.getMessageRepository().save(msg);
+            }
+        }
+    }
+
     protected void receive(ObjectMessage object, Broadcast broadcast) throws IOException {
         byte[] tag = broadcast instanceof V5Broadcast ? ((V5Broadcast) broadcast).getTag() : null;
         for (BitmessageAddress subscription : ctx.getAddressRepository().getSubscriptions(broadcast.getVersion())) {
diff --git a/core/src/main/java/ch/dissem/bitmessage/ProofOfWorkService.java b/core/src/main/java/ch/dissem/bitmessage/ProofOfWorkService.java
index e886c22..d242b79 100644
--- a/core/src/main/java/ch/dissem/bitmessage/ProofOfWorkService.java
+++ b/core/src/main/java/ch/dissem/bitmessage/ProofOfWorkService.java
@@ -12,6 +12,8 @@ import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
 import java.util.List;
+import java.util.Timer;
+import java.util.TimerTask;
 
 import static ch.dissem.bitmessage.InternalContext.NETWORK_EXTRA_BYTES;
 import static ch.dissem.bitmessage.InternalContext.NETWORK_NONCE_TRIALS_PER_BYTE;
@@ -29,14 +31,21 @@ public class ProofOfWorkService implements ProofOfWorkEngine.Callback, InternalC
     private MessageRepository messageRepo;
 
     public void doMissingProofOfWork() {
-        List<byte[]> items = powRepo.getItems();
+        final List<byte[]> items = powRepo.getItems();
         if (items.isEmpty()) return;
 
-        LOG.info("Doing POW for " + items.size() + " tasks.");
-        for (byte[] initialHash : items) {
-            Item item = powRepo.getItem(initialHash);
-            cryptography.doProofOfWork(item.object, item.nonceTrialsPerByte, item.extraBytes, this);
-        }
+        // Wait for 30 seconds, to let the application start up before putting heavy load on the CPU
+        new Timer().schedule(new TimerTask() {
+            @Override
+            public void run() {
+                LOG.info("Doing POW for " + items.size() + " tasks.");
+                for (byte[] initialHash : items) {
+                    Item item = powRepo.getItem(initialHash);
+                    cryptography.doProofOfWork(item.object, item.nonceTrialsPerByte, item.extraBytes,
+                            ProofOfWorkService.this);
+                }
+            }
+        }, 30_000);
     }
 
     public void doProofOfWork(ObjectMessage object) {
@@ -79,7 +88,6 @@ public class ProofOfWorkService implements ProofOfWorkEngine.Callback, InternalC
                 messageRepo.save(plaintext);
             }
             ctx.getInventory().storeObject(object);
-            powRepo.removeObject(initialHash);
             ctx.getNetworkHandler().offer(object.getInventoryVector());
         } else {
             item.message.getAckMessage().setNonce(nonce);
@@ -96,6 +104,7 @@ public class ProofOfWorkService implements ProofOfWorkEngine.Callback, InternalC
             }
             doProofOfWork(item.message.getTo(), object);
         }
+        powRepo.removeObject(initialHash);
     }
 
     @Override
diff --git a/core/src/main/java/ch/dissem/bitmessage/entity/Plaintext.java b/core/src/main/java/ch/dissem/bitmessage/entity/Plaintext.java
index a2b66f7..b90bf14 100644
--- a/core/src/main/java/ch/dissem/bitmessage/entity/Plaintext.java
+++ b/core/src/main/java/ch/dissem/bitmessage/entity/Plaintext.java
@@ -16,6 +16,7 @@
 
 package ch.dissem.bitmessage.entity;
 
+import ch.dissem.bitmessage.entity.payload.Msg;
 import ch.dissem.bitmessage.entity.payload.Pubkey;
 import ch.dissem.bitmessage.entity.valueobject.InventoryVector;
 import ch.dissem.bitmessage.entity.valueobject.Label;
@@ -511,7 +512,7 @@ public class Plaintext implements Streamable {
                 to = new BitmessageAddress(0, 0, destinationRipe);
             }
             if (type == Type.MSG && ackMessage == null && ackData == null) {
-                ackData = cryptography().randomBytes(32);
+                ackData = cryptography().randomBytes(Msg.ACK_LENGTH);
             }
             return new Plaintext(this);
         }
diff --git a/core/src/main/java/ch/dissem/bitmessage/entity/payload/GenericPayload.java b/core/src/main/java/ch/dissem/bitmessage/entity/payload/GenericPayload.java
index 4ae4696..176d938 100644
--- a/core/src/main/java/ch/dissem/bitmessage/entity/payload/GenericPayload.java
+++ b/core/src/main/java/ch/dissem/bitmessage/entity/payload/GenericPayload.java
@@ -53,6 +53,10 @@ public class GenericPayload extends ObjectPayload {
         return stream;
     }
 
+    public byte[] getData() {
+        return data;
+    }
+
     @Override
     public void write(OutputStream stream) throws IOException {
         stream.write(data);
diff --git a/core/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java b/core/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java
index edc4643..64a010c 100644
--- a/core/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java
+++ b/core/src/main/java/ch/dissem/bitmessage/entity/payload/Msg.java
@@ -33,6 +33,7 @@ import static ch.dissem.bitmessage.entity.Plaintext.Type.MSG;
  */
 public class Msg extends ObjectPayload implements Encrypted, PlaintextHolder {
     private static final long serialVersionUID = 4327495048296365733L;
+    public static final int ACK_LENGTH = 32;
 
     private long stream;
     private CryptoBox encrypted;
diff --git a/core/src/main/java/ch/dissem/bitmessage/factory/Factory.java b/core/src/main/java/ch/dissem/bitmessage/factory/Factory.java
index 49db9d6..53e1676 100644
--- a/core/src/main/java/ch/dissem/bitmessage/factory/Factory.java
+++ b/core/src/main/java/ch/dissem/bitmessage/factory/Factory.java
@@ -23,6 +23,8 @@ import ch.dissem.bitmessage.entity.Plaintext;
 import ch.dissem.bitmessage.entity.payload.*;
 import ch.dissem.bitmessage.entity.valueobject.PrivateKey;
 import ch.dissem.bitmessage.exception.NodeException;
+import ch.dissem.bitmessage.utils.TTL;
+import ch.dissem.bitmessage.utils.UnixTime;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -210,6 +212,6 @@ public class Factory {
         if (plaintext == null || plaintext.getAckData() == null)
             return null;
         GenericPayload ack = new GenericPayload(3, plaintext.getFrom().getStream(), plaintext.getAckData());
-        return new ObjectMessage.Builder().objectType(MSG).payload(ack).build();
+        return new ObjectMessage.Builder().objectType(MSG).payload(ack).expiresTime(UnixTime.now(+TTL.msg())).build();
     }
 }
diff --git a/core/src/main/java/ch/dissem/bitmessage/ports/MessageRepository.java b/core/src/main/java/ch/dissem/bitmessage/ports/MessageRepository.java
index c4cff5c..0c46499 100644
--- a/core/src/main/java/ch/dissem/bitmessage/ports/MessageRepository.java
+++ b/core/src/main/java/ch/dissem/bitmessage/ports/MessageRepository.java
@@ -34,6 +34,8 @@ public interface MessageRepository {
 
     Plaintext getMessage(byte[] initialHash);
 
+    Plaintext getMessageForAck(byte[] ackData);
+
     List<Plaintext> findMessages(Label label);
 
     List<Plaintext> findMessages(Status status);
diff --git a/demo/src/test/java/ch/dissem/bitmessage/SystemTest.java b/demo/src/test/java/ch/dissem/bitmessage/SystemTest.java
index 6751819..03cbf5e 100644
--- a/demo/src/test/java/ch/dissem/bitmessage/SystemTest.java
+++ b/demo/src/test/java/ch/dissem/bitmessage/SystemTest.java
@@ -5,11 +5,13 @@ import ch.dissem.bitmessage.entity.BitmessageAddress;
 import ch.dissem.bitmessage.entity.Plaintext;
 import ch.dissem.bitmessage.networking.DefaultNetworkHandler;
 import ch.dissem.bitmessage.ports.DefaultLabeler;
+import ch.dissem.bitmessage.ports.Labeler;
 import ch.dissem.bitmessage.repository.*;
 import ch.dissem.bitmessage.utils.TTL;
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.mockito.Mockito;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -20,6 +22,7 @@ import static ch.dissem.bitmessage.entity.payload.Pubkey.Feature.DOES_ACK;
 import static ch.dissem.bitmessage.utils.UnixTime.MINUTE;
 import static org.hamcrest.CoreMatchers.equalTo;
 import static org.junit.Assert.assertThat;
+import static org.mockito.Matchers.any;
 
 /**
  * @author Christian Basler
@@ -29,6 +32,7 @@ public class SystemTest {
 
     private BitmessageContext alice;
     private TestListener aliceListener = new TestListener();
+    private Labeler aliceLabeler = Mockito.spy(new DebugLabeler("Alice"));
     private BitmessageAddress aliceIdentity;
 
     private BitmessageContext bob;
@@ -53,7 +57,7 @@ public class SystemTest {
                 .networkHandler(new DefaultNetworkHandler())
                 .cryptography(new BouncyCryptography())
                 .listener(aliceListener)
-                .labeler(new DebugLabeler("Alice"))
+                .labeler(aliceLabeler)
                 .build();
         alice.startup();
         aliceIdentity = alice.createIdentity(false, DOES_ACK);
@@ -93,6 +97,9 @@ public class SystemTest {
 
         assertThat(plaintext.getType(), equalTo(Plaintext.Type.MSG));
         assertThat(plaintext.getText(), equalTo(originalMessage));
+
+        Mockito.verify(aliceLabeler, Mockito.timeout(TimeUnit.MINUTES.toMillis(15)).atLeastOnce())
+                .markAsAcknowledged(any());
     }
 
     @Test
diff --git a/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcHelper.java b/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcHelper.java
index 1df12f6..beb4c80 100644
--- a/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcHelper.java
+++ b/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcHelper.java
@@ -18,6 +18,7 @@ package ch.dissem.bitmessage.repository;
 
 import ch.dissem.bitmessage.entity.Streamable;
 import ch.dissem.bitmessage.entity.payload.ObjectType;
+import ch.dissem.bitmessage.exception.ApplicationException;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
@@ -25,6 +26,7 @@ import java.io.ByteArrayOutputStream;
 import java.io.IOException;
 import java.sql.PreparedStatement;
 import java.sql.SQLException;
+import java.util.Collection;
 
 import static ch.dissem.bitmessage.utils.Strings.hex;
 
@@ -85,4 +87,12 @@ public abstract class JdbcHelper {
             ps.setBytes(parameterIndex, os.toByteArray());
         }
     }
+
+    protected <T> T single(Collection<T> collection) {
+        if (collection.size() > 1) {
+            throw new ApplicationException("This shouldn't happen, found " + collection.size() +
+                    " messages, one or none was expected");
+        }
+        return collection.stream().findAny().orElse(null);
+    }
 }
diff --git a/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcMessageRepository.java b/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcMessageRepository.java
index ae1b496..ffc639f 100644
--- a/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcMessageRepository.java
+++ b/repositories/src/main/java/ch/dissem/bitmessage/repository/JdbcMessageRepository.java
@@ -137,16 +137,12 @@ public class JdbcMessageRepository extends JdbcHelper implements MessageReposito
 
     @Override
     public Plaintext getMessage(byte[] initialHash) {
-        List<Plaintext> plaintexts = find("initial_hash=X'" + Strings.hex(initialHash) + "'");
-        switch (plaintexts.size()) {
-            case 0:
-                return null;
-            case 1:
-                return plaintexts.get(0);
-            default:
-                throw new ApplicationException("This shouldn't happen, found " + plaintexts.size() +
-                        " messages, one or none was expected");
-        }
+        return single(find("initial_hash=X'" + Strings.hex(initialHash) + "'"));
+    }
+
+    @Override
+    public Plaintext getMessageForAck(byte[] ackData) {
+        return single(find("ack_data=X'" + Strings.hex(ackData) + "' AND status='" + Plaintext.Status.SENT + "'"));
     }
 
     @Override
@@ -174,7 +170,7 @@ public class JdbcMessageRepository extends JdbcHelper implements MessageReposito
         try (
                 Connection connection = config.getConnection();
                 Statement stmt = connection.createStatement();
-                ResultSet rs = stmt.executeQuery("SELECT id, iv, type, sender, recipient, data, sent, received, status " +
+                ResultSet rs = stmt.executeQuery("SELECT id, iv, type, sender, recipient, data, ack_data, sent, received, status " +
                         "FROM Message WHERE " + where)
         ) {
             while (rs.next()) {
@@ -187,6 +183,7 @@ public class JdbcMessageRepository extends JdbcHelper implements MessageReposito
                 builder.IV(new InventoryVector(iv));
                 builder.from(ctx.getAddressRepository().getAddress(rs.getString("sender")));
                 builder.to(ctx.getAddressRepository().getAddress(rs.getString("recipient")));
+                builder.ackData(rs.getBytes("ack_data"));
                 builder.sent(rs.getLong("sent"));
                 builder.received(rs.getLong("received"));
                 builder.status(Plaintext.Status.valueOf(rs.getString("status")));
@@ -268,8 +265,8 @@ public class JdbcMessageRepository extends JdbcHelper implements MessageReposito
 
     private void insert(Connection connection, Plaintext message) throws SQLException, IOException {
         try (PreparedStatement ps = connection.prepareStatement(
-                "INSERT INTO Message (iv, type, sender, recipient, data, sent, received, status, initial_hash) " +
-                        "VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?)",
+                "INSERT INTO Message (iv, type, sender, recipient, data, ack_data, sent, received, status, initial_hash) " +
+                        "VALUES (?, ?, ?, ?, ?, ?, ?, ?, ?, ?)",
                 Statement.RETURN_GENERATED_KEYS)
         ) {
             ps.setBytes(1, message.getInventoryVector() == null ? null : message.getInventoryVector().getHash());
@@ -277,10 +274,11 @@ public class JdbcMessageRepository extends JdbcHelper implements MessageReposito
             ps.setString(3, message.getFrom().getAddress());
             ps.setString(4, message.getTo() == null ? null : message.getTo().getAddress());
             writeBlob(ps, 5, message);
-            ps.setLong(6, message.getSent());
-            ps.setLong(7, message.getReceived());
-            ps.setString(8, message.getStatus() == null ? null : message.getStatus().name());
-            ps.setBytes(9, message.getInitialHash());
+            ps.setBytes(6, message.getAckData());
+            ps.setLong(7, message.getSent());
+            ps.setLong(8, message.getReceived());
+            ps.setString(9, message.getStatus() == null ? null : message.getStatus().name());
+            ps.setBytes(10, message.getInitialHash());
 
             ps.executeUpdate();
             // get generated id
diff --git a/repositories/src/main/resources/db/migration/V3.2__Update_table_message.sql b/repositories/src/main/resources/db/migration/V3.2__Update_table_message.sql
new file mode 100644
index 0000000..8b43b0a
--- /dev/null
+++ b/repositories/src/main/resources/db/migration/V3.2__Update_table_message.sql
@@ -0,0 +1 @@
+ALTER TABLE Message ADD COLUMN ack_data BINARY(32);