[TriLUG] to whom do I send SMTP?

[Richard O. Hammer]

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?

(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?

I have noticed that I can do 2).  With my DSL account with
Sprint/Earthlink, the mailserver mail.earthlink.net always
seems to accept the test messages which I send out through
it.  But I wonder if that is the normal and the right way.  I
have tried 1) a few times, with limited success, so I wonder
if 1) is the normal and correct way.

Thank you,
Rich Hammer