3. How Email Works? Outbound MTA (Queue) Inbound MTA Mailstore (mailboxes) POP/IMAP Send mail via MTA (using SMTP) Fetch mails from mailstore (using POP/IMAP) Route mails to their destination Internal Email

7. SMTP Protocol Replies Permanent Negative Completion reply. e.g. 500 Syntax error, command unrecognized 5yz Transient Negative Completion reply. E.g. 452 Requested action not taken: insufficient system storage 4yz Positive Intermediate reply. E.g. 354 Start mail input; end with <CRLF>.<CRLF> 3yz Positive Completion reply. e.g. 221 <domain> Service closing transmission channel 2yz Positive Preliminary reply. 1yz

8. Envelope splitting Envelope (as received): Rcpt to: gtom@mirapoint.com Rcpt to: [email_address] Rcpt to: [email_address] Rcpt to: a [email_address] Internal recipient Envelope (to mx1.mirapoint.com): Rcpt to: gtom@mirapoint.com Rcpt to: [email_address] Envelope (to mx04.hotmail.com): Rcpt to: [email_address]

11. SMTP Extensions

15. POP3 States & Commands TOP msg n, UIDL [msg], USER, PASS, APOP Optional POP3 Commands QUIT UPDATE STAT, LIST [msg], RETR msg, DELE msg, NOOP, RSET TRANSACTION QUIT AUTHORIZATION Commands State

17. Typical POP Session

21. POP3 Vs IMAP Complexity Protocol granularity Append, Flags Folders Typical Use Feature IMAP protocol is complex which make client and server implementation is demanding task. IMAP clients are not that widely available especially on the compact devices Less complex which makes it easy to implement pop client and server. POP clients are available on wide variety of devices including most of the compact devices Retrieve any sub part of the mail, search Limited retrieve entire mail or headers, no search Yes No Yes No Access mails from multiple machines Access mails from single machine IMAP POP3

23. IMAP Extensions