I posted a similar question a week ago, but the problem has reoccurred.
Conditions:
Windows NT domain, Exchange 5.5, Outlook 2000 clients.
Symptoms:
Cannot send email to some domains (ie anyone at somecompany.com or someisp.com) – the Exchange server returns an NDR stating “Host Unreachable”.
I can sit at the Exchange server and telnet to the mail servers of those domains and see the ESMTP connection.
Since I can telnet there, DNS lookups are working and IP connectivity is OK.The problem I had before was unchanged after replacing the server, the firewall, the switch, the patch cable, the internet connection, the client PC, and the Outlook version. The problem has also just – in the last 3 days – shown up on another Exchange server at another office (different company)
When we removed the firewall and plugged the Exchange server into the internet directly, all mail that was in the outbound queue was sent, very quickly. After putting the server behind the firewall it still worked for a few days but is now failing again.
The firewall is a Cisco PIX with SMTP wide open. We saw the same problem (no change) with a Sonicwall, also having SMTP wide open.
We noticed some probes coming _from_ port 25 on remote servers _to_ random ports on our server.
help… please…