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The number two cause of Sendmail installation problems is
due to a firewall or other network security device blocking your
HP e3000 from being able to send and receive packets on port 53
(DNS) and port 25 (SMTP).
Sendmail uses port 53 (DNS) to resolve hostnames into IP addresses
and IP addresses into hostnames. Sendmail may do multiple DNS resolutions
for every e-mail message sent or received, and if a firewall is blocking
these DNS packets, Sendmail may experience long delays and/or generate
various error messages logged to syslog.
Sendmail may need to contact external DNS servers if you are
attempting to exchange e-mail with the Internet. Some intranet environments
may require you to reference a "forwarding DNS server" (which
can traverse your border firewall to talk to the Internet) via a
nameserver statement in /SYS/NET/RESLVCNF. Consult your local network
administrator for advice on how to choose a proper DNS server.
Port 25 (SMTP) is used to connect to remote mail servers to
deliver outgoing e-mail, and is also used on the HP e3000 to listen
for incoming e-mail. If a firewall is blocking outbound port 25
packets, Sendmail may experience long delays and generate various
error messages logged to syslog as well as bounce messages returned
to the e-mail originator. If a firewall is blocking inbound port
25 packets, Sendmail will not be able to receive any incoming e-mail,
and there will be no extra syslog messages.
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