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 +139,19 @@
 
             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])['domain']
+            if not userdomain is None:
+              realm = userdomain
+              self.domain = userdomain
         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 +624,14 @@
 
     return user
 
+def user_get_domain(user=None):
+    if user == 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()