One thing to consider is that when your SMTP demon makes a connection to
your recipients mail hub, there is an exchange between the two
computers, beginning with your computer saying HELO (or EHLO) and then
identifying itself. Many mailservers reject mail if the announced name
does not match the name received by DNS lookup. What this means is that
if you don't have a static IP address (which you won't on a dialup line)
there will be some hosts that you cannot send mail to. Using a
smarthost, which has a static IP and passes the reverse DNS check, will
allow you to send mail to these hosts.
HTH,
Steve