I am running MS Exchange 5.5 connected via an ISDN line. Most mail goes but 25-40 per day end up in the outbound mail queue getting rescheduled, with “Host Unreachable” as the error. The ISDN line is busy/overloaded, but if I try to manually re-send/reschedule the queue when the line is not busy, the mail still errors with “Host Unreachable”. The addresses are good – I can send new mail, but I cannot get items in the queue to send. Is there a DNS timeout/retry in Exchange? The only similarity on the emails that don’t go – similar hosts, often yahoo or hotmail addresses, but many other hosts as well.
