Sunday night (no one around) our Domino 7 server suddenly stopped sending outbound SMTP messages. Notes mail routes correctly.
Inbound SMTP mail is received.
POP and IMAP work fine.
When I put a packet sniffer (Ethereal) on the box, no packets go on the wire when the server console displayes the error (for a yahoo.com example):
"No messages transferred to YAHOO.COM (host c.mx.mail.YAHOO.COM) via SMTP: The server is not responding. The server may be down or you may be experiencing network problems. Contact your system administrator if this problem persists."
So it looks like DNS is working too.
From Windows2003 (server's OS), I can ping address but I cannot telnet over port 25 to addresses.
An interesting (and possibly relevant) footnote: Symantec Antivirus cannot run its LiveUpdate from the console on that machine with the following error: "LU1814: LiveUpdate could not retrieve the update list". The description goes on to suggest that we check to make sure the machine has Internet access.