Sometimes, my LDAP server is not responding for a couple of seconds and when Postfix does its lookup, it bounces with an error: "Invalid user specified". Is there a way to force the lookup to be retried a few seconds later before bouncing?
What about just increasing the timeout for LDAP lookups? The ldap_table man page has details. The default is 10 seconds. If it's not a timeout issue -- that is, if the LDAP server is actually rejecting connections -- maybe you could set up a local replica on the Postfix server. Whether or not this is possible depends a lot on your environment and which server you're running.