By default, sendmail on RedHat and Fedora accepts connectios only from 127.0.0.1. You have to change that.
If you have webmin installed, go to Webmin->Servers->
Sendmail->Sendmail Options. Under "SMTP Port Options" you often find the following entry:
RH: Port=smtp, Addr=127.0.0.1, Name=MTA
Please change it to "Default" and then click on "Save and Apply". And also have a look at your firewall settings...