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SMTP Commands Guide
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- What is SMTP
- SMTP handles email sending, receiving, and relaying between client and server
- SMTP server performs verifications and transfers emails to recipient's inbox
- IMAP protocol enables recipients to download emails from receiving servers
- ESMTP Features
- ESMTP extends SMTP with multimedia file sharing capabilities
- Includes authentication, SSL encryption, and multiple recipient support
- Allows simultaneous transmission and size restrictions
- SMTP Session Structure
- Handshake establishes TCP connection between client and server
- Email transfer occurs after authentication checks
- Termination closes TCP connection between client and server
- Key Commands
- HELO/EHLO initiates SMTP session and client-server greeting
- AUTH command verifies client identity
- MAIL FROM/RCPT TO specify sender and recipient addresses
- DATA/BDAT commands transfer email content
- VRFY confirms username/mailbox existence
- RSET resets connection without severing
- QUIT terminates session
- HELP command provides command reference
- Response Codes
- SMTP responses use three-digit numeric codes
- First digit indicates command success or failure
- Second digit shows response type
- Third digit provides additional command information