diff --git a/INSTALL b/INSTALL
index 49aae922a..14bf27b94 100644
--- a/INSTALL
+++ b/INSTALL
@@ -19,6 +19,7 @@ REQUIREMENTS
    - MDB2 2.5.0 or newer
    - Mail_Mime 1.8.1 or newer
    - Net_SMTP 1.4.2 or newer
+   - Net_IDNA2 0.1.1 or newer
    - Auth_SASL 1.0.3 or newer
 * php.ini options (see .htaccess file):
    - error_reporting E_ALL & ~E_NOTICE (or lower)