Introduction to SMTP SMTP stands for simple mail transfer protocol and is documented in RFC5321 but it has been standardized some 30 years ago stared long long ago. Such a long time ago when Internet was just in its infancy and email was still being considered a hobby project. It is a text based project a la HTTP and it has only 5 or so commands. HELO/EHLO MAIL FROM RCPT TO DATA QUIT And the status codes also match the standard HTTP codes in what it means to have a 2xx or 4xx response.
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