diff --git a/Mailbox-Properties-Client.md b/Mailbox-Properties-Client.md
index fae6822..2702550 100644
--- a/Mailbox-Properties-Client.md
+++ b/Mailbox-Properties-Client.md
@@ -11,7 +11,9 @@ 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 of the `properties` dictionary are strings.
+**UPDATE** - The message body is a BDF list with 4 elements: `version` (int), `properties` (dictionary), `briarSupports` (list), and `mailboxSupports` (list).
+
+`version` is incremented whenever the properties change. The keys of the `properties` dictionary are strings.
 
 For major version 0, the `properties` dictionary of a well-formed update contains either no (0) entries, which means for the receiver that there is no mailbox available. Or, it contains all of the following entries:
 
@@ -20,6 +22,10 @@ For major version 0, the `properties` dictionary of a well-formed update contain
 * "inboxId" : byte[32] -- folder id used when receiving data from the mailbox
 * "outboxId" : byte[32] -- folder id used when delivering data to the mailbox
 
+`briarSupports` is a list of Mailbox versions that this Briar instance supports (can communicate with). It must not be empty. Each element is a list with 2 elements: `maj` (int), `min` (int).
+
+`mailboxSupports` is a list of Briar versions that the mailbox supports. It must be empty if `properties` is empty (there is no mailbox). Each element is a list with 2 elements: `maj` (int), `min` (int).
+
 ### Validity policy
 
 * An update is valid if it is well-formed.