SMTP
- TCP 25
- client-server, RFC 822
- reply codes:
- 2**: success
- 250 = OK
- 3**: redirection
- 254 = send next chunk
- 4**: temporary failure
- 452 = mailbox full
- 5**: permanent failure
- 550 = fatal error
- 2**: success
- DNS MX contains mail server FQDN and priority (lower – better)
- DNS A for domain – fallback if MX record not found
- server puts email (data) into envelope that is responsible for delivery
- commands
- HELO/EHLO <src domain>
- MAIL FROM
- RCPT TO
- DATA: end of message marked by line with ‘.’ only
- STARTTLS
- QUIT