Add AEAP transition (#3385)

spec.md

@@ -1,6 +1,6 @@
 # chat-mail specification
 
-Version: 0.33.0
+Version: 0.34.0
 Status:  In-progress 
 Format:  [Semantic Line Breaks](https://sembr.org/)
 
@@ -474,4 +474,20 @@ as the sending time of the message as indicated by its Date header,
 or the time of first receipt if that date is in the future or unavailable.
 
 
+# Transitioning to a new e-mail address (AEAP)
+
+When receiving a message:
+- If the key exists, but belongs to another address
+- AND there is a `Chat-Version` header
+- AND the message is signed correctly
+- AND the From address is (also) in the encrypted (and therefore signed) headers 
+- AND the message timestamp is newer than the contact's `lastseen`
+  (to prevent changing the address back when messages arrive out of order)
+  (this condition is not that important
+  since we will have eventual consistency even without it):
+
+  Replace the contact in _all_ groups,
+  possibly deduplicate the members list,
+  and add a system message to all of these chats.
+
 Copyright © 2017-2021 Delta Chat contributors.