diff --git a/Mailbox-Properties-Client.md b/Mailbox-Properties-Client.md
index 3cae87d..0ebbe9a 100644
--- a/Mailbox-Properties-Client.md
+++ b/Mailbox-Properties-Client.md
@@ -13,7 +13,14 @@ The client uses a separate BSP group for communicating with each contact. The [g
 
 ### Message types
 
-**UPDATE** - The message body is a BDF list with two elements: `version` (int), and `properties` (dictionary). `version` is incremented whenever the properties change. The keys and values of the `properties` dictionary are strings. Typical keys in this dictionary are: *onionAddress*, *token*, *inboxId*, *outboxId*. An empty dictionary would mean for the receiver that there is no mailbox available for use.
+**UPDATE** - The message body is a BDF list with two elements: `version` (int), and `properties` (dictionary). `version` is incremented whenever the properties change. The keys of the dictionary are strings.
+
+For major version 0, the dictionary of a well-formed update is either empty, which means for the receiver that there is no mailbox available for use. Or it contains precisely 4 items:
+
+* "onionPubKey" : byte[32]
+* "token"       : byte[32]
+* "inboxId"     : byte[32]
+* "outboxId"    : byte[32]
 
 ### Validity policy