[TriLUG] to whom do I send SMTP?

Richard O. Hammer ROHammer at EarthLink.net
Mon Sep 30 14:32:32 EDT 2002


Thank you, Jon.  That clears it up considerably, even though
you've told me there is no single correct answer (Sendmail may
try several servers according to some scheme).

I don't think my ISP limits my communication through my node
on the Internet; I've got a dynamically assigned IP address. 
I seem to be able to telnet to port 25 of most any mail
server, and at least attempt an SMTP exchange.


Rich



Jon Carnes wrote:
> 
> On Mon, 2002-09-30 at 13:18, Richard O. Hammer wrote:
> > I am working on a service which needs to send SMTP messages.
> > With all the concern about improper forwarding of email
> > messages, I want to know: to which SMTP server is it proper
> > for me to send a given message?
> 
> If you are working from a internet connection where the ISP limits the
> forwarding of SMTP, then you will need to use the ISP's SMTP servers...
> Most user based ISP's do block SMTP at the border. If they provide a
> business class solution, then they may or may not also allow you
> straight access to other smtp servers that are off of their net.
> 
> This is a royal pain to business folks that travel.  The best solution
> is to use Authenticated SMTP (on a high port).
> >
> > (Here is a similar question, worded another way) When Sendmail
> > sends a message to rover at dog.org, does it:
> > 1) query which mailserver serves dog.org, and then connect to
> > that mailserver to send rover's message? or
> > 2) connect to a mailserver run by the ISP, from which this
> > Sendmail-hosting organization buys its Internet connection,
> > and send the message to rover at dog.org through that server?
> 
> By default Sendmail does a DNS based MX lookup to find the proper
> mailserver, and then attempts a direct connection. If that fails, it
> moves on to the secondary. Most small/medium businesses use their ISP's
> mailservers as a secondary (or tertiary).
> 
> So the mail may not be delivered directly, but sendmail does find a
> responsible party to handle the mail.
> 
> If a company is running off an SMTP restrictive ISP and the company does
> not arrange for the ISP to act as an SMTP secondary, then Sendmail will
> most likely not be able to deliver the mail.
> 
> You can of course, set sendmail to use the ISP's SMTP servers as relays.
> 
> Jon
> 
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