Vanishing outgling mail

[Mike Keithley]

I'm using Slackware 4.0 and have sendmail installed.  I use fetchmail to
get mail from the POP3 server of my ISP and this works great.  I do a
PPP connection to the ISP.

My problem is that outgoing mail simply vanishes.  I know the SMTP
machine I should use on my ISP but don't know how to tell sendmail what
it is.

Can any one help? Thanks.