As documented in the RFCs, SMTP (Simple Mail Transfer Protocol) usually starts by
HELO (short for "hello") or EHLO commands, if I'm not mistaken. These commands are used to indicate
to the mail server which machine is trying to connect (used to update the headers of your e-mails, where a list
of all machines that have handled the mail is stored).
But, the SMTP protocol is very simple. Just Google around and you'll find your answer, I'm sure of it.
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