SMTPServer

A simple SMTP server which stores your mails on your file system.

How to use it

1. Execute
   SMTPServer.java.

2. Select a networking utility of your choice (e.g. netcat or telnet) and run, let's say,
   telnet localhost 8000.

3. Use the implemented SMTP-commands in the list below and have fun!

Implemented SMTP-Commands

  - HELO <SP> <domain> <CRLF>

  - MAIL <SP> FROM:<reverse-path> <CRLF>

  - RCPT <SP> TO:<forward-path> <CRLF>

  - DATA <CRLF>

  - HELP [<SP> <string>] <CRLF>

  - QUIT <CRLF>

Reply codes by function groups (not completely implemented)

  - 500 Syntax error, command unrecognized 
  [This may include errors such as command line too long]
  - 501 Syntax error in parameters or arguments
  - 502 Command not implemented
  - 503 Bad sequence of commands
  - 504 Command parameter not implemented

---

  - 211 System status, or system help reply
  - 214 Help message 
  [Information on how to use the receiver or the meaning of a particular non-standard command;
  this reply is useful only to the human user]

---

  - 220 \<domain> Service ready
  - 221 \<domain> Service closing transmission channel
  - 421 \<domain> Service not available, closing transmission channel 
  [This may be a reply to any command if the service knows it must shut down]

---

  - 250 Requested mail action okay, completed
  - 251 User not local; will forward to \<forward-path>
  - 450 Requested mail action not taken: mailbox unavailable [E.g., mailbox busy]
  - 550 Requested action not taken: mailbox unavailable [E.g., mailbox not found, no access]
  - 451 Requested action aborted: error in processing
  - 551 User not local; please try \<forward-path>
  - 452 Requested action not taken: insufficient system storage
  - 552 Requested mail action aborted: exceeded storage allocation
  - 553 Requested action not taken: mailbox name not allowed
  [E.g., mailbox syntax incorrect]
  - 354 Start mail input; end with \<CRLF>.\<CRLF>
  - 554 Transaction failed

Command-Reply sequences

  S success
  F failure
  E error
  I intermediate

  CONNECTION ESTABLISHMENT
     S: 220
     F: 421
  HELO
     S: 250
     E: 500, 501, 504, 421
  MAIL
     S: 250
     F: 552, 451, 452
     E: 500, 501, 421
  RCPT
     S: 250, 251
     F: 550, 551, 552, 553, 450, 451, 452
     E: 500, 501, 503, 421
  DATA
     I: 354 -> data -> S: 250
                       F: 552, 554, 451, 452
     F: 451, 554
     E: 500, 501, 503, 421
  HELP
     S: 211, 214
     E: 500, 501, 502, 504, 421
  QUIT
     S: 221
     E: 500

For further reading, check out https://datatracker.ietf.org/doc/html/rfc821/