On Thu, Jan 09, 2003 at 12:30:58PM +0200, Matthew Fincham wrote:
> A telnet to the mailer fails (no route to host) - but while a ping is
> successfully running (very confusing).

As Steven Bellovin mentioned, this might mean that port 25 is being
blocked by a firewall... it's possible that your ISP wants you to use 
their SMTP server for your outgoing mail, rather than trying to contact 
destination mailservers directly. (Prevents their customers from
scanning for open SMTP servers and sending spam through them).

> It seems like it is all "external" (I'm not sure what that means) servers
> which fail. I created a Yahoo mail account and tried to send mail to it,
> from both my NetBSD (using mail) and Windows (using Outlook Express) and the
> NetBSD version failed with a "no route to host" error. Yet these two systems
> are on the same network. So I think it is some setting on the NetBSD
> machine. Maybe a hosts issue??

What did you set as the SMTP server in Outlook Express? Try using the
same server as your "smart host" in /etc/mail/sendmail.cf (look for the
"DS" line... E.g., change it to something like:

# "Smart" relay host (may be null)
DSmailserver.myisp.com