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Why gui mail clients need smtp

server

(editor's note): A question was asked as to why mail clients (Outlook, Eudora, etc) insist upon a "smart host" : an smtp server to deliver mail to.

Many answers were given, but this is actually the one that makes sense.

Subject: Re: What is the use of SMTP server
References: <3eb007f1.0307162102.25cb4bc8@posting.google.com> 
From: Johan Kullstam <kullstj-nn@comcast.net>
Date: Sun, 20 Jul 2003 13:29:29 GMT

sonyantony@hotmail.com (Sony Antony) writes:

> I was playing with sendmail and .forward file on my RH Linux system.
> When I added an email to the .forward file, sendmail faithfully sent
> it to the destination.
> But then I realized that nowhere had I specified my SMTP server.

From the RFC on e-mail (what is it 2833?)
SMTP client sends mail.
SMTP server receives mail.  

Sometimes the server can receive mail for others and relay it to its
final destination.

> So I started investigating using strace ing sendmail.
> I found out that sendmail did not use an SMTP server at all. It did an
> MX query, got the mail gateway for the destination address domain, and
> sent the mail directly.

Sure it did.  Sendmail (as client) talked directly to the server at
the other end.  This is how e-mail usually works.

You can also relay it through the server at your ISP.  Check into
the "smarthost" configuration for your local sendmail.

> This makes me think why ISPs give an SMTP server and ask you to
> configure your mail client to point to that.

Unfortunately, many people have improperly set up systems.  This has
led to semi-widespread blocking of people coming from dynamic IP
assignments.  Thus you might be pretty well forced to relay through
your ISP's SMTP server.

> Why is that none of the mail clients like evolution, are built with
> the kind of logic as sendmail.

Evolution is a Mail User Agent (MUA) and expects a Mail Transfer Agent
(MTA), e.g., sendmail to do the queuing and retransmission &c.

> Why is that users are asked to do one more configuration step (
> configuring SNTP server for the mail client ) that can be
> eliminated.

Well, if the destination host is down, you might want the mail system
to retry in a few minutes/hours rather than just not working.  Usually
the task of ensuring delivery is delegated to a dedicated program for
that task rather than your reader.  This lets you choose the pretty
reader independently of choosing the reliable delivery mechanism.

> Any pointers will be appreciated.
> --sony

-- 
Johan KULLSTAM

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