diff --git a/README b/README
index 05ef0d1c8aa2e0f2d7cee328b9b0b07630ff4a2e..9d46b01e76c1ab08827ec653789093a46be9b25f 100644
--- a/README
+++ b/README
@@ -101,6 +101,7 @@ following list outlines the implementation status of each supported extension:
     imap4flags: full (old imapflags supported for backwards compatibility)
     variables: mostly full; currently no support for future namespaces 
     notify: full, mailto support only; needs to be tested more
+    mailbox: almost full, but mailboxexists command lacks ACL support
 
 All implemented extensions are like the engine itself currently experimental. 
 A status of 'full' does not necessarily mean that the extension is bug-free or