Php mail, how to solve this problem-php Tutorial

Php Mail has an introduction to php & nbsp; Principles of sending mails? (not to the php mail class & nbsp; is it a post about principles & nbsp; not found locally & nbsp; SMTP server & nbsp;) ------ solution ------------------ sending an email is actually based on the & nbsp; SMTP protocol. it is very easy to send an email in php.
Is there an introduction to php mail principles?

(Not the class for sending mails to php. it is a post about the principle. there is no SMTP server locally)

------ Solution --------------------
Sending an email is actually based on the SMTP protocol. it is very simple, just a few words to understand

HELO a.com
Mail from: <[email protected]>
Rcpt to: <[email protected]>
DATA
SUBJECT: KAKAKKSGL

This is content
.

Then use the socket series functions in PHP.