--- a/sat/memory/encryption.py	Mon Mar 23 16:55:15 2020 +0100
+++ b/sat/memory/encryption.py	Mon Mar 23 17:52:18 2020 +0100
@@ -442,23 +442,38 @@
         to_jid = mess_data['to']
         encryption = self._sessions.get(to_jid.userhostJID())
         if encryption is not None:
-            if mess_data["type"] == "groupchat" and encryption['plugin'].directed:
+            plugin = encryption['plugin']
+            if mess_data["type"] == "groupchat" and plugin.directed:
                 raise exceptions.InternalError(
                 f"encryption flag must not be set for groupchat if encryption algorithm "
                 f"({encryption['plugin'].name}) is directed!")
             mess_data[C.MESS_KEY_ENCRYPTION] = encryption
-            self.markAsEncrypted(mess_data)
+            self.markAsEncrypted(mess_data, plugin.namespace)
 
     ## Misc ##
 
-    def markAsEncrypted(self, mess_data):
+    def markAsEncrypted(self, mess_data, namespace):
         """Helper method to mark a message as having been e2e encrypted.
 
         This should be used in the post_treat workflow of messageReceived trigger of
         the plugin
         @param mess_data(dict): message data as used in post treat workflow
+        @param namespace(str): namespace of the algorithm used for encrypting the message
         """
         mess_data['extra'][C.MESS_KEY_ENCRYPTED] = True
+        from_bare_jid = mess_data['from'].userhostJID()
+        if from_bare_jid != self.client.jid.userhostJID():
+            session = self.getSession(from_bare_jid)
+            if session is None:
+                # if we are currently unencrypted, we start a session automatically
+                # to avoid sending unencrypted messages in an encrypted context
+                log.info(_(
+                    "Starting e2e session with {peer_jid} as we receive encrypted "
+                    "messages")
+                    .format(peer_jid=from_bare_jid)
+                )
+                defer.ensureDeferred(self.start(from_bare_jid, namespace))
+
         return mess_data
 
     def isEncryptionRequested(self, mess_data, namespace=None):