The Simple Mail Transfer Protocol is ancient, but too widely used to change easily. It is connection-oriented, like the TCP Protocol. It is commonly used in email servers for sending email or between e-mail servers.
Commands
- MAIL: Establishes return address, bounce address, sender, and more
- RCPT: Establishes address of recipient, repeated per recipient
- DATA: Signifies start of message text with header and body
| SMTP Code | Meaning |
|---|---|
| 220 | SMTP Service Ready |
| 250 | Requested Action Completed |
| 421 | Service Unavailable |
| 450 | User’s Mailbox Unavailable |
| 451 | Recipient’s Server Error |
| 452 | Server Storage Insufficient |
| 500 | Command Syntax Error |
| 501 | Command Arguments Syntax Error |
| 503 | Bad Sequence of Commands |
| 550 | User’s Mailbox Unabailable |
| 551 | Recipient Not Local |
| 552 | Recipient’s Email Full |
| 554 | Delivery Error |