diff --git a/lib/Auth.php b/lib/Auth.php
index 701c385..f91198a 100644
--- a/lib/Auth.php
+++ b/lib/Auth.php
@@ -244,6 +244,11 @@ class Auth {
         return $this->auth_instance()->domainadmin_get_configuration($domain, $variablename);
     }
 
+    public function domainadmin_get_user_quota($domainadmin, $excludeuser)
+    {
+        return $this->auth_instance()->domainadmin_get_user_quota($domainadmin, $excludeuser);
+    }
+
     public function find_recipient($address)
     {
         return $this->auth_instance()->find_recipient($address);
diff --git a/lib/Auth/LDAP.php b/lib/Auth/LDAP.php
index 3ac1f43..f105e5f 100644
--- a/lib/Auth/LDAP.php
+++ b/lib/Auth/LDAP.php
@@ -638,6 +638,31 @@ class LDAP extends Net_LDAP3 {
         return array('domainadmin' => $maindomainadmin, $config_flag => $resultvalue);
     }
 
+    // get the quota that has already been shared among the users of the domains that this domainadmin manages.
+    // excluding the current quota for the specified user, that we want to change the quota for
+    public function domainadmin_get_user_quota($domainadmin, $excludeuser)
+    {
+        $quota = 0;
+        $domains = $this->domainadmin_get_domains($domainadmin);
+        foreach ($domains as $domain) {
+            // get all users that are part of this domain
+            $users_result = $this->_search(
+                               $this->_standard_root_dn($domain),
+                               "objectclass=kolabinetorgperson",
+                               array($this->conf->get("quota_attribute")));
+            if ($users_result != null && count($users_result) > 0) {
+               $users = $users_result->entries(true);
+               foreach ($users as $uid => $user) {
+                   if (strtolower($uid) != strtolower($excludeuser)) {
+                       $quota += $user[$this->conf->get("quota_attribute")];
+                   }
+               }
+            }
+        }
+
+        return $quota;
+    }
+
     public function domain_edit($domain, $attributes, $typeid = null)
     {
         $domain = $this->domain_info($domain, array_keys($attributes));
diff --git a/lib/api/kolab_api_service_form_value.php b/lib/api/kolab_api_service_form_value.php
index 79a4720..f0d99ec 100644
--- a/lib/api/kolab_api_service_form_value.php
+++ b/lib/api/kolab_api_service_form_value.php
@@ -1237,7 +1237,66 @@ class kolab_api_service_form_value extends kolab_api_service
 
     private function validate_mailquota($value, $postdata = array(), $validation_type = null)
     {
-        return $this->validate_quota($value, $postdata, $validation_type);
+        $value = $this->validate_quota($value, $postdata, $validation_type);
+
+        if (empty($value)) {
+            return $value;
+        }
+
+        if (!is_numeric($value)) {
+            throw new Exception('Invalid value for mail quota. Please clear or enter a valid integer number!');
+        }
+
+        $value = intval($value);
+
+        $conf = Conf::get_instance();
+        $quota_attribute = $conf->get('domainadmin_quota_attribute');
+        if ($quota_attribute == null || strlen($quota_attribute) == 0) {
+          $quota_attribute = $conf->get('quota_attribute');
+        }
+        // check domain admin quota, only for users that are not part of the default domain
+        if (strtolower($postdata['ou']) != strtolower($conf->get('kolab_user_base_dn'))) {
+            $auth = Auth::get_instance();
+            
+            // get the mailquota of the domain admin for the current domain
+            $result = $auth->domainadmin_get_configuration($_SESSION['user']->get_domain(), $quota_attribute);
+
+            if (isset($result)) {
+                $domainadminquota = $result[$quota_attribute];
+                $domainadmin = $result['domainadmin'];
+                // get all quotas from all users of that domain admin, excluding this user
+                $quota_used = $auth->domainadmin_get_user_quota($domainadmin, 'uid='.$postdata['uid'].','.$postdata['ou']);
+
+                // check if existing quota plus this new quota would still fit the quota of the domain admin
+                if ($quota_used + $value > $domainadminquota) {
+                    $available = $domainadminquota - $quota_used;
+                    $domainadminquotaunit = "KB";
+                    if ($domainadminquota > 1024) {
+                        $domainadminquota = $domainadminquota / 1024;
+                        $domainadminquotaunit = "MB";
+                    }
+                    if ($domainadminquota > 1024) {
+                        $domainadminquota = $domainadminquota / 1024;
+                        $domainadminquotaunit = "GB";
+                    }
+                    $availableunit = "KB";
+                    if ($available > 1024) {
+                        $available = $available / 1024;
+                        $availableunit = "MB";
+                    }
+                    if ($available > 1024) {
+                        $available = $available / 1024;
+                        $availableunit = "GB";
+                    }
+
+                    throw new Exception('error: mailquota of the domain admin has been exceeded. <br/>'.
+                         'max available: '.$domainadminquota.' '.$domainadminquotaunit.';<br/> '.
+                         'max available for this user: '.$available.' '.$availableunit);
+                }
+            }
+        }
+
+        return (string) intval($value);
     }
 
     private function validate_tbitskolaboverallquota($value, $postdata = array(), $validation_type = null)