Application Layer

2.4 Electronic Mail in the Internet
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Introduction
2.1 Principles of Network Applications
2.1.1 Network Application Architectures
2.1.2 Processes Communcating
2.1.3 Application-Layer Protocols
2.1.4 What Services Does an Application Need?
2.1.5 Services Provided by the Internet Transport Protocols
2.2 The Web and HTTP
2.2.1Overview of HTTP
2.2.2 Nonpersistent and Persistent Connections
2.2.3 HTTP Message Format
2.2.4 User-Server Ineraction: Cookies
2.2.5 HTTP Content
2.2.6 Web Caching
2.2.7 The Conditional GET
2.3 File Transfer: FTP
2.3.1 FTP Commands and Replies
2.4 Electronic Mail in the Internet
2.4.1 STMP
2.4.2 Comparison with HTTP
2.4.3 Mail Message Formats and MIME
2.4.4 Mail Access Protocols
2.5 DNS--The Internet's Directory Service
2.5.1 Services Provided by DNS
2.5.2 Overview of How DNS Works
2.5.3 DNS Records and Messages
2.6 P2P File Sharing
2.7 Socket Programming with TCP
2.7.1 Socket Programming with TCP
2.7.2 An Example Client/Server Application in Java
2.8 Socket Programming with UDP

2.4.1 SMTP

SMTP is at the heart of Internet electronic mail.  SMTP transfers messages from senders' mail servers to the recipients' mail servers.  SMTP is much older than HTTP.  Although SMTP has numerous wounderful qualities it is nevertheless a legacy technology that possesses certain archaic characteristics. 
 
In the multimedia era, the 7-bit ASCII restriction is a bit of a pain it requires binary multimedia data to be enclosed to ASCII before being sent over SMTP; and it requires the corresponding ASCII message to be decoded back to binary after SMTP transport.
 
It is important to observe that SMTP does not normally use intermediate mail servers for sending mail, even when tthe two mail servers are located at opposite ends of the world.