I cannot send and/or receive emails.
Please check if Sendmail/Postfix and your POP3 daemon are running by executing
You will get a list of all services listening on your server.
If they are running re-configure your firewall (iptables or ipchains) so that the ports 25 (Sendmail/Postfix) and 110 (POP3) are not blocked. If this does not help please check that Sendmail/Postfix is really running on port 25 and your POP3 daemon on port 110.
(Hint for standard RedHat installations: If you have webmin installed and are running Sendmail, 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.)