How does “sendmail” send mails to any domain name?

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http://serverfault.com – I noticed that one of our servers (running Ubuntu) can send mails to any domain (yahoo.com ,gmail.com) using simple "sendmail" command. But I cannot figure out how to configure similar setup in a new server. I cannot see any files such as "/etc/mail/sendmail.cf" in the first server. How does this work ? Does it use some other SMTP server to do actual mail delivery? Where can I find these settings? (HowTos)