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Some helpful links:
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:
There are two solutions for this problem:
1) You configure
your MTA to allow authentication via SMTP (SMTP-Auth). Your users then
have to change settings in their email clients (e.g. Outlook), i.e.,
they have to activate Server requires authentication.
2) You install "POP-before-SMTP" or poprelayd on your server. Your users then have to fetch emails first before they are allowed to send emails for a restricted period of time (mostly 30 min.).