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# 21.16. `smtpd` — SMTP Server

Source code: Lib/smtpd.py

This module offers several classes to implement SMTP (email) servers.

Several server implementations are present; one is a generic do-nothing implementation, which can be overridden, while the other two offer specific mail-sending strategies.

Additionally the SMTPChannel may be extended to implement very specific interaction behaviour with SMTP clients.

The code supports RFC 5321, plus the RFC 1870 SIZE extension.

## 21.16.1. SMTPServer Objects

class `smtpd.``SMTPServer`(localaddr, remoteaddr, data_size_limit=33554432)

  Create a new `SMTPServer` object, which binds to local address localaddr. It will treat remoteaddr as an upstream SMTP relayer. It inherits from `asyncore.dispatcher`, and so will insert itself into `asyncore`‘s event loop on instantiation.

  data_size_limit specifies the maximum number of bytes that will be accepted in a `DATA` command. A value of `None` or `0` means no limit.

  `process_message`(peer, mailfrom, rcpttos, data)

    Raise `NotImplementedError` exception. Override this in subclasses to do something useful with this message. Whatever was passed in the constructor as remoteaddr will be available as the `_remoteaddr` attribute. peer is the remote host’s address, mailfrom is the envelope originator, rcpttos are the envelope recipients and data is a string containing the contents of the e-mail (which should be in RFC 2822 format).

  `channel_class`

    Override this in subclasses to use a custom `SMTPChannel` for managing SMTP clients.

## 21.16.2. DebuggingServer Objects

class `smtpd.``DebuggingServer`(localaddr, remoteaddr)

  Create a new debugging server. Arguments are as per `SMTPServer`. Messages will be discarded, and printed on stdout.

## 21.16.3. PureProxy Objects

class `smtpd.``PureProxy`(localaddr, remoteaddr)

  Create a new pure proxy server. Arguments are as per `SMTPServer`. Everything will be relayed to remoteaddr. Note that running this has a good chance to make you into an open relay, so please be careful.

## 21.16.4. MailmanProxy Objects

class `smtpd.``MailmanProxy`(localaddr, remoteaddr)

  Create a new pure proxy server. Arguments are as per `SMTPServer`. Everything will be relayed to remoteaddr, unless local mailman configurations knows about an address, in which case it will be handled via mailman. Note that running this has a good chance to make you into an open relay, so please be careful.

## 21.16.5. SMTPChannel Objects

class `smtpd.``SMTPChannel`(server, conn, addr)

  Create a new `SMTPChannel` object which manages the communication between the server and a single SMTP client.

  To use a custom SMTPChannel implementation you need to override the `SMTPServer.channel_class` of your `SMTPServer`.

  The `SMTPChannel` has the following instance variables:

  `smtp_server`

    Holds the `SMTPServer` that spawned this channel.

  `conn`

    Holds the socket object connecting to the client.

  `addr`

    Holds the address of the client, the second value returned by socket.accept()

  `received_lines`

    Holds a list of the line strings (decoded using UTF-8) received from the client. The lines have their “\r\n” line ending translated to “\n”.

  `smtp_state`

    Holds the current state of the channel. This will be either `COMMAND` initially and then `DATA` after the client sends a “DATA” line.

  `seen_greeting`

    Holds a string containing the greeting sent by the client in its “HELO”.

  `mailfrom`

    Holds a string containing the address identified in the “MAIL FROM:” line from the client.

  `rcpttos`

    Holds a list of strings containing the addresses identified in the “RCPT TO:” lines from the client.

  `received_data`

    Holds a string containing all of the data sent by the client during the DATA state, up to but not including the terminating “rn.rn”.

  `fqdn`

    Holds the fully-qualified domain name of the server as returned by `socket.getfqdn()`.

  `peer`

    Holds the name of the client peer as returned by `conn.getpeername()` where `conn` is `conn`.

  The `SMTPChannel` operates by invoking methods named `smtp_<command>` upon reception of a command line from the client. Built into the base `SMTPChannel` class are methods for handling the following commands (and responding to them appropriately):

  Command | Action taken
  --- | ---
  HELO | Accepts the greeting from the client and stores it in `seen_greeting`. Sets server to base command mode.
  EHLO | Accepts the greeting from the client and stores it in `seen_greeting`. Sets server to extended command mode.
  NOOP | Takes no action.
  QUIT | Closes the connection cleanly.
  MAIL | Accepts the “MAIL FROM:” syntax and stores the supplied address as `mailfrom`. In extended command mode, accepts the RFC 1870 SIZE attribute and responds appropriately based on the value of `data_size_limit`.
  RCPT | Accepts the “RCPT TO:” syntax and stores the supplied addresses in the `rcpttos` list.
  RSET | Resets the `mailfrom`, `rcpttos`, and `received_data`, but not the greeting.
  DATA | Sets the internal state to `DATA` and stores remaining lines from the client in `received_data` until the terminator “rn.rn” is received.
  HELP | Returns minimal information on command syntax
  VRFY | Returns code 252 (the server doesn’t know if the address is valid)
  EXPN | Reports that the command is not implemented.

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