This document discusses the Simple Mail Transfer Protocol (SMTP). It covers topics such as user agents and addresses that send and receive email, commands and responses used to transfer messages between mail servers, and extensions like MIME that allow adding attachments and formatting. Diagrams show how email is transferred between user agents, mail transfer agents, and mail gateways in a store-and-forward fashion using SMTP.

1. Chapter 22 Simple Mail Transfer Protocol (SMTP)

2. CONTENTS USER AGENT (UA) ADDRESSES DELAYED DELIVERY ALIASES MAIL TRANSFER AGENT (MTA) COMMANDS AND RESPONSES MAIL TRANSFER PHASES MIME MAIL DELIVERY MAIL ACCESS PROTOCOLS

3. Figure 22-1 SMTP concept

4. Figure 22-2 UAs and MTAs

5. Figure 22-3 Relay MTAs

6. Figure 22-4 Mail gateway

7. USER AGENT (UA) 22.1

8. Figure 22-5 Format

9. ADDRESSES 22.2

10. Figure 22-6 Address

11. DELAYED DELIVERY 22.3

12. Figure 22-7 Sender-site delay

13. Figure 22-8 Receiver-site delay

14. ALIASES 22.4

15. Figure 22-9 Alias expansion

16. Figure 22-10 One-to-many expansion

17. Figure 22-11 Many-to-one expansion

18. MAIL TRANSFER AGENT 22.5

19. Figure 22-12 Email system

20. COMMANDS AND RESPONSES 22.6

21. Figure 22-13 Commands and responses

22. Figure 22-14 Command format

23. MAIL TRANSFER PHASES 22.7

24. Figure 22-15 Connection establishment

25. Figure 22-16 Message transfer

26. Figure 22-17 Connection termination

27. MULTIPURPOSE INTERNET MAIL EXTENSIONS (MIME) 22.8

28. Figure 22-18 MIME

29. Figure 22-19 MIME header

30. Figure 22-20 Base64

31. Figure 22-21 Quoted-printable

32. MAIL DELIVERY 22.9

33. Figure 22-22 Email delivery

34. MAIL ACCESS PROTOCOLS 22.10

35. Figure 22-22 POP3