SMTP stands for Simple Mail Transport Protocol. It is the method that computers connected to the Internet use to send email. (Your "Outgoing Mail" settings in your email program can also be referred to as your SMTP settings.) It is also the method that servers use to transfer email between them.
SMTP transactions typically have 4 parts:
- HELO, where the computers talking identify themselves
- MAIL FROM, the envelope sender of the message is given
- RCPT TO, the address or addresses that the message will be sent to
- DATA, the actual message (which also has all the message headers, including From: and To:)
Many spam filters, including most of Pobox's, run after HELO, MAIL FROM and RCPT TO, but before DATA. That's because, once you accept the DATA, you can no longer bounce the message. That is why any filters that run on the message content, including filters you set up yourself, cannot bounce mail.