I hope you meant "telnet into 127
.0.0.1 on port 25"?
You need to make the rc.sendmail file executable so that Sendmail will be started during the boot of your computer.
chmod +x /etc/rc.d/rc.sendmail
Next time, post your Slackware related question in the Slackware forum where it will be seen by a much larger crowd of experienced slackers.