sendmail configuration on redhat 2003-01-17 - By James P. Roberts
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> Hi Jim
>
> For my own clarification , is there a difference between Handling POP3
> and SMTP requests , and actually BEING a POP3 / SMTP server ?
Not really. Pretty much the definition of a "server," I should think.
>
> The reason I ask is that I have a whole bunch of Windows machines all
> connected to my Red Hat linux mail server running sendmail , and in
the
> Outlook express account settings , I have the incoming mail ( POP3 )
> and outgoing mail ( SMTP ) all pointing to the IP address of my mail
> server. If sendmail is not a POP server , how does it handle the
> incoming mail ?
A POP server handles delivery of locally stored ("already delivered to
the destination SMTP server") email, to a requesting client (such as
Outlook). On Redhat, this service is typically provided by the ipop3d
daemon. (More accurately, it is typically the xinetd daemon, which
listens on the POP3 port, and wakes up the ipop3d daemon as needed.) It
can send emails to Outlook (for example), but does not accept emails
from Outlook.
Sendmail provides SMTP service, which means it can receive mail for
delivery to a local mailbox, relay email to another SMTP server, or send
outgoing mails to another SMTP server. It can receive emails from
Outlook (for example), but it does not send emails to Outlook.
The magic of multiple servers running on one machine (same IP address)
is handled by "port numbers." There are, if I recall correctly, some
64,000+ port numbers defined for each IP address (it's a built-in part
of the IP protocol). This, incidentally, is a good lead in to a
discussion of firewalls, which typically block some of these ports, or
define rules for what kind of traffic is allowed on certain ports.
Really quite fascinating!
I hope this is helpful?
Jim
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