Are you doing LDAP verification from within Postfix or are you using Postfix to blacklist abusive/dictionary Ips?
The most efficient way to block the dictionary attacks is to periodically export the user list from the mailbox server to the MX, so the MX can reject invalid users from its local copy of legit users.
postfix is so efficient in rejecting in invalid recipients (I've seen 20 rejects/second with no strain on postfix), there's no need to blacklist the abusive IPs.
However, IMGate "advanced" does harvest the abusive PTR-less IPs and Class C's into a postfix database so those IPs and Class C's are permanently blacklisted, so they can't ever send to legit users.
Len
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