SMTP .SMTP protocol is a standard Internet protocol, which is used to transmit e-mail from sender server to recipient server and client computer. SMTP protocol stipulates the basic rules and conventions of mail transmission, including the format of mail header, the size limit of mail, the delivery mode of mail and so on.

SMTP protocol is implemented by TCPIP protocol stack, and SMTP service is identified by port number 25. SMTP protocol supports authentication mechanism to ensure that only authorized users can access SMTP server and send mail. SMTP protocol also supports mail distribution function, can send mail to multiple recipients, and supports the transmission of mail attachments.

SMTP protocol is a very important part of e-mail system, which ensures that e-mail can be safely and reliably transmitted to the destination and stored and processed correctly on the receiver server and client computer.