Simple Mail Transfer Protocol

2. The SMTP Model 2.1 Basic Structure 2.2 The Extension Model 2.2.1 Background 2.2.2 Definition and Registration of Extensions 2.3 Terminology 2.3.1 Mail Objects 2.3.2 Senders and Receivers 2.3.3 Mail Agents 2.3.4 Host 2.3.5 Domain 2.3.6 Buffer and State Table 2.3.7 Lines 2.3.8 Originator, Delivery, Relay, and Gateway Systems 2.3.9 Message Content and Mail Data 2.3.10 Mailbox and Address 2.3.11 Reply 2.4 Syntax Principles 2.4.1 General Syntax and Transaction Model 2.4.2 Command and Reply Syntax