A hacker doesn't really need PHP to send an Email. He needs only a command prompt and a telnet program, that's all. But we are in the PHP Zone, so we will do it with PHP too. All the stuff here is for learning purpose. Don't use it to hack or to spam or to do any other illegal action.
First, a good start is to read the SMTP specification protocol. It helps to understand how things work. You can read the
Request For Comments number 2821 document (RFC2821) for that. It's a good idea to read the entire document, but if you haven't the time, just read the chapter 4.
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