Hi, Let me explain the situation further. One customers domain is hit with hundreds of thousands of spam messages to random non existing recipient addresses from random sender addresses in bursts that last a few hours.
Recipient verify probes clog every filtering nodes' mail queues with tens of thousands verify probes and that effectively stops legitimate mails getting through to all other clients too until the verify probes are dealt with. The customer has Exchange and with co-operation with the customer tarpit was dropped from the incoming receiver and connection concurrency lifted from the default 20 connections to 200. We also changed transport for the client's verify probes. Any mentioned configuration change didn't help us much to cope with the problem of clogging mail queues, only the period of clogging was decreased to approximately one third, which is still more than can be tolerated. Obtaining and keeping valid recipient address lists up to date with a few thousand domains is not an option due to work load issues involved. We don't use sender verify because all we really can do is to make sure the senders domain exists. Sender verify also has problems coping with greylisting and tarpitting at least. Looks like someone else has also hit this problem earlier http://serverfault.com/questions/312962/postfix-connection-cache-address-verification-probes BR, Mika
