General Use:-
SMTP is generally used to transfer messages from a client to a server.So the user needs to
specify both POP or IMAP server and the SMTP server when the user configures the e-mail
application.Client applications use either a Post Office Protocol or the Internet Message
Access or a proprietary system to access their mail accounts on a mail-server.

Overview of the protocol:-
This protocol consists of the following three command reply sequences:-
i) Mail Command to establish the return address of the message.
ii) RCPT Command to establish a recipient of the message.This command can be issued
multiple times.
iii) Data to send the message text.This is the content of the message.

Limitation of SMTP:-
SMTP can transfer mail only.It cannot receive mails from a distant server.
Connection:-
Connection to an SMTP with the server occurs with the help of a port,usually port 25.
Use of an ISP’s SMTP Server:-Users who are outside ISP’s network are not allowed to
access the server.