SMTP协议解析

Electronic mail is undoubtedly one of the most popular application .Shows that about one-half of all TCP connections are for the Simple Mail Transfer Protocol, SMTP.

The communication between the two MTAs uses NVT ASCII. Commands are sent by the client to the server , and the server responds with numeric reply codes and optional human-readable strings.

Simple Example

Envelopes, Headers, and Body

Relay Agents

NVT ASCII

Retry intervals

We showed normal mail delivery in the previous section, so here we'll show how MX records are used for mail delivery and illustrate the VRFY and EXPN commans.

MX Records: Hosts Not Directly Connected to the Internet

MX Records: Hosts That Are Down

VRFY and EXPN Commands

Changes are taking place with Internet mail . Recall the three pieces that comprise Internet mail: the envelope, headers and body .

Envelope Changes: Extend SMTP

Header Changes: Non-ASCII Characters

Body Changes: Multipurpose Internet Extensions