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You can use PHP's built in mail system, which just sends the messages to the recipient is gets from a form (mostly). This is a very basic method, but works most of the time.
You can also use the SMTP method, that uses a mailserver to relay all messages through. This requires the mailserver allows your site to use it as a mail relay, and with the current spam issues, not many mailservers do that. If, however, you have control over a mailserver, this can be a very nice option to use.
Some SMTP servers require you to authenticate with your suername and password (and allow you to relay messages when logged in), so that's where SMTP Auth is for.
Hope this helps!
Herko
