For an hour or so today our email server was rejecting email. The server would respond with an error that the account does not exist. It turns out that it could not look up users in LDAP because one of our LDAP servers was having a problem handling the load. The LDAP server thought it had too many open files. To fix the problem we have added another LDAP server and also added an idletimeout of 60 seconds. Hopefully that will help us handle the load and keep the LDAP servers functioning properly.