diff --git a/pom.xml b/pom.xml
index efb6329..a7a89ac 100644
--- a/pom.xml
+++ b/pom.xml
@@ -6,7 +6,7 @@
 
     <groupId>org.example</groupId>
     <artifactId>Widerhall</artifactId>
-    <version>0.1.2</version>
+    <version>0.1.3</version>
     <build>
         <plugins>
             <plugin>
diff --git a/src/main/java/de/srsoftware/widerhall/data/MailingList.java b/src/main/java/de/srsoftware/widerhall/data/MailingList.java
index aa2cd3a..7cc28ff 100644
--- a/src/main/java/de/srsoftware/widerhall/data/MailingList.java
+++ b/src/main/java/de/srsoftware/widerhall/data/MailingList.java
@@ -7,6 +7,8 @@ import de.srsoftware.widerhall.mail.SmtpClient;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import javax.mail.Address;
+import javax.mail.Flags;
 import javax.mail.Message;
 import javax.mail.MessagingException;
 import javax.mail.internet.AddressException;
@@ -15,6 +17,7 @@ import java.io.UnsupportedEncodingException;
 import java.sql.ResultSet;
 import java.sql.SQLException;
 import java.util.*;
+import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
 import static de.srsoftware.widerhall.Constants.*;
@@ -46,6 +49,7 @@ public class MailingList implements MessageHandler {
     private static final int VISIBLE = 1;
     private static final int HIDDEN = 0;
     private static final int DEFAULT_STATE = STATE_PENDING|STATE_HIDE_RECEIVERS;
+    private static final String RETAINED_FOLDER = "retained";
     private final String name;
     private final String email;
     private int state;
@@ -136,13 +140,9 @@ public class MailingList implements MessageHandler {
 
 
     public MailingList enable(boolean enable) throws SQLException {
+        if (!enable) imap.stop();
         setFlag(STATE_ENABLED,enable);
-
-        if (enable) {
-            imap.start().addListener(this);
-        } else {
-            imap.stop();
-        }
+        if (enable) imap.start().addListener(this);
         return this;
     }
 
@@ -217,7 +217,7 @@ public class MailingList implements MessageHandler {
      * @return
      */
     public boolean isOpenFor(User user) {
-        if ((state & STATE_PUBLIC) > 0) return true; // all users may subscribe public mailing lists
+        if (hasState(STATE_PUBLIC)) return true; // all users may subscribe public mailing lists
         if (user == null) return false;
         try {
             var member = ListMember.load(this,user);
@@ -312,10 +312,33 @@ public class MailingList implements MessageHandler {
             } catch (SQLException e){
                 LOG.error("Was not able to load members of {}; Non-Delivery notification dropped!",this.email(),e);
             }
-        } else {
-            storeMessage(message);
-            forward(message);
+            return;
+        }
+
+        Address from = message.getFrom()[0];
+        if (from instanceof InternetAddress internetAddress){
+            var senderEmail = ((InternetAddress) from).getAddress();
+            if (!hasState(STATE_OPEN) && !this.hashMember(senderEmail)){
+                retainMessage(message);
+                sentRetentionNotification(senderEmail);
+                return;
+            }
         }
+        storeMessage(message);
+        forward(message);
+    }
+
+
+
+
+    private boolean hashMember(String senderEmail) {
+        if (senderEmail == null) return false;
+        try {
+            return members().stream().map(ListMember::user).map(User::email).anyMatch(senderEmail::equals);
+        } catch (SQLException e) {
+            LOG.warn("hasMember() failded for {}",email(),e);
+        }
+        return false;
     }
 
     public MailingList open(boolean open) throws SQLException {
@@ -346,7 +369,29 @@ public class MailingList implements MessageHandler {
         return setFlag(STATE_REPLY_TO_LIST,on);
     }
 
+    private void retainMessage(Message message)  {
+        String subject = "unknown mail";
+        try {
+            subject = message.getSubject();
+            imap.move(message, RETAINED_FOLDER);
+            return;
+        } catch (MessagingException e){
+            LOG.warn("Retaining message {} failed!",subject,e);
+        }
+        try {
+            message.setFlag(Flags.Flag.SEEN, true);
+            return;
+        } catch (MessagingException e) {
+            LOG.warn("Failed to flag message {} as SEEN!",subject,e);
+        }
 
+        try {
+            LOG.error("Retaining message {} failed. To avoid dead loop, the MailingList '{}' will be stopped!",subject,email());
+            enable(false);
+        } catch (SQLException sqle) {
+            LOG.debug("Failed to update list state in database:",sqle);
+        }
+    }
     /**
      * creates a map of the current ML containing all fields but passwords.
      * @return
@@ -402,7 +447,30 @@ public class MailingList implements MessageHandler {
         var config = Configuration.instance();
         var url = new StringBuilder(config.baseUrl()).append("/confirm?token=").append(token);
         var text = t("Please go to {} in order to complete your list subscription!",url);
-        smtp.login().send(email(),name(),user.email(),subject,text);
+        smtp.send(email(),name(),user.email(),subject,text);
+    }
+
+    private void sentRetentionNotification(String senderEmail) {
+        try {
+            var receivers = members()
+                    .stream()
+                    .filter(ListMember::isOwner)
+                    .map(ListMember::user)
+                    .map(User::email)
+                    .collect(Collectors.joining(", "));
+            var subject = t("List '{}' requires attention!",name());
+            var text = t("This list received an email from {}, who is not member of the list.\nThe email has been moved to the '{}' folder.\nYou may manually forward this message or drop it.",senderEmail,RETAINED_FOLDER);
+            smtp.send(email(), name(), receivers,subject,text);
+
+            subject = t("Your message to {} was rejected!",email());
+            text = t("You have tried to send a message to the list '{}', which failed. This is because you are not a member of this list.\n",name());
+            if (hasState(STATE_PUBLIC)) text += t("You may go to {} and subscribe to the list, then try again.",Configuration.instance().baseUrl());
+            smtp.send(email(), name(), senderEmail,subject,text);
+        } catch (SQLException e){
+            LOG.error("Failed to load list of owners of mailing list. Retention notification was not sent to owners of {}",email(),e);
+        } catch (MessagingException | UnsupportedEncodingException e){
+            LOG.error("Failed to send retention notification to owners of {}",email(),e);
+        }
     }
 
     private MailingList setFlag(int flag, boolean on) throws SQLException {
@@ -416,6 +484,10 @@ public class MailingList implements MessageHandler {
         if (hasState(STATE_ENABLED)) map.put("enabled",VISIBLE);
         if (hasState(STATE_PUBLIC)) map.put("public",VISIBLE);
         if (hasState(STATE_FORWARD_FROM)) map.put("original_from",HIDDEN);
+        if (hasState(STATE_FORWARD_ATTACHED)) map.put("forward_attached",HIDDEN);
+        if (hasState(STATE_HIDE_RECEIVERS)) map.put("hide_receivers",HIDDEN);
+        if (hasState(STATE_REPLY_TO_LIST)) map.put("reply_to_list",HIDDEN);
+        if (hasState(STATE_OPEN)) map.put("open",VISIBLE);
         return map;
     }
 
diff --git a/src/main/java/de/srsoftware/widerhall/mail/ImapClient.java b/src/main/java/de/srsoftware/widerhall/mail/ImapClient.java
index 0c0c2a7..e1d58ce 100644
--- a/src/main/java/de/srsoftware/widerhall/mail/ImapClient.java
+++ b/src/main/java/de/srsoftware/widerhall/mail/ImapClient.java
@@ -23,6 +23,7 @@ public class ImapClient {
         private static final Logger LOG = LoggerFactory.getLogger(ListeningThread.class);
         private HashSet<MessageHandler> listeners = new HashSet<>();
         private boolean stopped = false;
+        private Session session;
 
         public ListeningThread addListener(MessageHandler messageHandler) {
             listeners.add(messageHandler);
@@ -53,7 +54,7 @@ public class ImapClient {
         private void openInbox() throws MessagingException {
             LOG.debug("Connecting and logging in…");
             Properties props = imapProps();
-            Session session = Session.getInstance(props);
+            session = Session.getInstance(props);
             Store store = session.getStore(Constants.IMAPS);
             store.connect(host,username,password);
             LOG.debug("Connected, opening {}:",folderName);
@@ -75,6 +76,8 @@ public class ImapClient {
             for (Message message : inbox.getMessages()){
                 if (message.isSet(Flags.Flag.SEEN)) continue;
                 handle(message);
+                Folder folder = message.getFolder();
+                if (!folder.isOpen()) folder.open(Folder.READ_WRITE);
                 message.setFlag(Flags.Flag.SEEN,true);
             }
         }
@@ -158,6 +161,21 @@ public class ImapClient {
         return folderName;
     }
 
+    public ImapClient move(Message message, String destinationFolder) throws MessagingException {
+        if (listeningThread == null || listeningThread.stopped) throw new IllegalStateException("IMAP client not connected!");
+        var source = message.getFolder();
+        if (!source.isOpen()) source.open(Folder.READ_WRITE);
+        var messages = new Message[]{message};
+        var store = source.getStore();
+        var dest = store.getFolder(new URLName(destinationFolder));
+        if (!dest.exists()) dest.create(Folder.HOLDS_MESSAGES);
+        source.copyMessages(messages,dest);
+        source.setFlags(messages, new Flags(Flags.Flag.DELETED), true);
+        source.close(true);
+        return this;
+    }
+
+
     public ImapClient start() {
         stop();
 
diff --git a/src/main/java/de/srsoftware/widerhall/mail/SmtpClient.java b/src/main/java/de/srsoftware/widerhall/mail/SmtpClient.java
index 2c0f875..7f0a043 100644
--- a/src/main/java/de/srsoftware/widerhall/mail/SmtpClient.java
+++ b/src/main/java/de/srsoftware/widerhall/mail/SmtpClient.java
@@ -92,6 +92,7 @@ public class SmtpClient {
     }
 
     public void send(String senderAdress, String senderName, String receivers, String subject, String content) throws MessagingException, UnsupportedEncodingException {
+        login();
         MimeMessage message = new MimeMessage(session);
         message.addHeader("Content-Type","text/plain; charset="+UTF8);
         message.addHeader("format","flowed");