[TriLUG] to whom do I send SMTP?

[Jon Carnes]

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