SMTP
Simple Mail Transfer Protocol for email transmission:
Message Format
| Field | Description | Example Value |
|---|---|---|
| Command | SMTP command | MAIL, RCPT, DATA, HELO, EHLO, QUIT |
| Parameters | Command arguments | FROM:<sender@domain.com>, TO:<recipient@domain.com> |
| Response Code | 3-digit status code | 250, 354, 550 |
| Response Text | Human-readable message | OK, Start mail input, Mailbox unavailable |
| Terminator | CRLF sequence | \r\n |
Response Codes
| Code | Category | Description |
|---|---|---|
2xx | Success | Command completed successfully |
3xx | Intermediate | Command accepted, more info needed |
4xx | Transient Error | Temporary failure, retry possible |
5xx | Permanent Error | Command failed, do not retry |