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In what ways can mail be sent? I am familiar with cron mail and php sending mail, but does a mail MTA such as postfix or sendmail need to be installed on the server? Is php also a mta? What if sendmail/postfix isn't installed?

I'm also curious about how web forms with emails send registration emails out to people. I suppose it could be php's 'mail' function or something else.

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PHP scripts can both use an SMTP server (such as a local MTA) or send the mail directly. It depends on the script. – Dennis Apr 18 '13 at 21:37

PHP is not an MTA. By default, mail() uses the local MTA, but you can configure PHP to use a remote MTA over SMTP, if none exists on the local machine.

Most web forms use mail() to send email, as might be expected; there are lots of message construction libraries out there, but they all pretty much boil down to mail() underneath.

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