SMTP, POP3, IMAP Protocols (en)
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Simple Explanation of How SMTP, POP3, and IMAP Protocols Work
Main Differences:
- SMTP: Used only for sending emails.
- POP3: Downloads emails to a local device and deletes them from the server (if configured that way).
- IMAP: Synchronizes emails between the server and various devices, allowing access from anywhere.
Simple Interaction Examples:
- Sending email: Using SMTP.
- Reading email: Using POP3 or IMAP.
- Retrieving email: Using IMAP (to download emails to a local device without deleting them from the server).
Note: The handshake examples above are a very simplified version. In practice, these protocols have many more complex commands and responses.
SMTP (Simple Mail Transfer Protocol)
- Function: Used for sending emails.
- How it works:
- The email client (e.g., Outlook, Gmail) sends an email to the SMTP server.
- The SMTP server checks the email, ensures the destination address is valid, and adds the necessary headers.
- The SMTP server forwards the email to the destination server.
- The destination server stores the email in the recipient's inbox.
- Example handshake:
Client: HELO mydomain.com Server: 250 mydomain.com, pleased to meet you Client: MAIL FROM:<[email address removed]> Server: 250 OK Client: RCPT TO:<[email address removed]> Server: 250 OK Client: DATA Server: 354 Go ahead Client: (email content) Client: . Server: 250 OK
POP3 (Post Office Protocol version 3)
- Function: Used for downloading emails from the server to the client device.
- How it works:
- The email client connects to the POP3 server.
- The client downloads all unread emails or selected emails.
- The client usually deletes emails from the server after downloading (unless configured to keep a copy).
- Example handshake:
Client: USER username Server: +OK Password required Client: PASS password Server: +OK user is authenticated Client: STAT Server: +OK 2 500 Client: RETR 1 Server: +OK 500 octets (Server sends the email content) Client: DELE 1 Server: +OK message 1 deleted Client: QUIT Server: +OK POP3 server signing off
IMAP (Internet Message Access Protocol)
- Function: Used for accessing emails from various devices. Emails remain stored on the server and can be accessed from anywhere.
- How it works:
- The email client connects to the IMAP server.
- The client can choose to download the entire email, just the headers, or only unread emails.
- Changes made to emails (e.g., marked as read, moved to another folder) will be synchronized with the server.
- Example handshake:
Client: A001 CAPABILITY Server: * CAPABILITY IMAP4rev1 CHILDREN IDLE NAMES STARTTLS UIDPLUS Server: A001 OK CAPABILITY completed Client: A002 LOGIN username password Server: A002 OK Logged in Client: A003 SELECT INBOX Server: * 1 EXISTS Server: * 2 RECENT Server: * FLAGS (\Seen \Answered \Flagged \Deleted \Draft) Server: A003 OK [READ-WRITE] Selected Client: A004 FETCH 1:2 ALL Server: * 1 FETCH (FLAGS (\Seen) INTERNALDATE "14-Aug-2012 10:45:01 +0100" RFC822 {...}) Server: * 2 FETCH (FLAGS (\Recent) INTERNALDATE "14-Aug-2012 10:46:01 +0100" RFC822 {...}) Server: A004 OK FETCH completed
Conclusion:
SMTP, POP3, and IMAP are complementary protocols in the email system. SMTP is responsible for sending, POP3 for downloading, and IMAP for synchronization and access from various devices.