I cannot send and/or receive emails.
Please check if Sendmail/Postfix and your POP3 daemon are running.
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 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".)