firewall question

[hanfamily at earthlink.net]

I bought a book on Linux firewalls and am working my way through
it. Unfortunately the examples are all written for someone with
a static IP address. I use dialup so my address changes each
time I log on. In their example for outgoing mail they
have
iptables -A OUTPUT -o $INTERNET -p tcp \
         -s $IPADDR --sport $UNPRIVPORTS \
         -d $SMTP_GATEWAY --dport 25 -m state --state NEW -j ACCEPT
Since my public ipaddr will change each time I log on would I just
use --sport $UNPRIVPORTS
also since earthlink only gives names not address for their smtp mail 
server would I drop the reference to SMTP_GATEWAY and just use
--dport 25 -m state --state NEW -j ACCEPT
I really appreciate all the help. 
             Linda