Electronic mail (e-mail) is the most used and most popular with users of an application on the Internet.
Simple Mail Transfer Protocol (SMTP)
Because the Internet SMTP can only transmit 7-bit ASCII messages can be printed, it is also proposed Common Internet Mail Extensions MIME .
MIME type data described in the header portion of the message which the message (e.g., text, voice, image, video, etc.). It can simultaneously transmit multiple types of message in the MIME
The data. This multimedia communications environment is very useful.
An e-mail system should have three primary components, that is the user agent, a mail server, and a mail transmission protocol (e.g., SMTP) and Mail Access Protocol (e.g., the POP3) .
The UA is the interface with the user's e-mail system, in most cases it is to run a program on the user's computer. Therefore, also known as a user agent
E-mail client software.
User agents should have at least the following four functions:
1, writing
2, display
3, treatment
4, communication
SMTP：
1, well-known port number SMTP connection establishment 25
2, mail delivery
3, connection release
Now commonly used e-mail to read the agreement there are two, namely the Post Office Protocol version 3 POP3 and Internet message access protocol IMAP .
Sender's user agent sends a message to the sender mail server and the sender mail server to send mail to a recipient mail server, the SMTP protocol is used.
And POP3 or IMAP protocol to read the message the user agent is used from the recipient mail server.