Why is your port 15 open? That's netstat, that's definitely a bad thing. Close it! Now! A hacker could get all the information on your network that they needed to lauch a full on attack.

Get rid of Telnet and install SSH. Telnet allows the transmission of cleartext passwords. Not a good thing.

As for FTP and SMPT these are known to have exploits in them. Read the sendmail bugfix list if you want some examples.

If it were my box, none of these ports would be open.