diff --git a/pykolab/auth/ldap/__init__.py b/pykolab/auth/ldap/__init__.py
--- a/pykolab/auth/ldap/__init__.py
+++ b/pykolab/auth/ldap/__init__.py
@@ -33,6 +33,7 @@
 from pykolab.constants import *
 from pykolab.errors import *
 from pykolab.translate import _
+from pykolab import wap_client
 
 log = pykolab.getLogger('pykolab.auth')
 conf = pykolab.getConf()
@@ -138,8 +139,24 @@
 
             Called from pykolab.auth.Auth, the realm parameter is derived, while
             login[3] preserves the originally specified realm.
-        """
 
+            If unique_uid_across_domains is defined as true,
+            and username is not an email address,
+            and the username is unique across all domains,
+            then the domain is determined that contains this username
+            and used as the realm
+        """
+
+        if conf.get('kolab', 'unique_uid_across_domains') == "true":
+            if ((not "@" in login[0])
+                    and (login[0] != 'cyrus-admin')
+                    and (realm == conf.get('kolab', 'primary_domain'))):
+                wap_client.authenticate()
+                userdomain = wap_client.user_get_domain(login[0])
+                if (userdomain is not False
+                        and not userdomain['domain'] is None):
+                    realm = userdomain['domain']
+                    self.domain = userdomain['domain']
         try:
             log.debug(
                     _("Attempting to authenticate user %s in realm %s") % (
diff --git a/pykolab/wap_client/__init__.py b/pykolab/wap_client/__init__.py
--- a/pykolab/wap_client/__init__.py
+++ b/pykolab/wap_client/__init__.py
@@ -624,6 +624,16 @@
 
     return user
 
+
+def user_get_domain(user=None):
+    if user is None:
+        user = utils.ask_question("User unique id")
+
+    _params = {'id': user}
+
+    return request('GET', 'user.get_domain', get=_params)
+
+
 def user_form_value_generate(params=None):
     if params == None:
         params = get_user_input()