# Programmatic usage

If you already have an asyncio event loop, you can create a server using the `SMTP` class as the protocol factory, and then run the loop forever. If you need to pass arguments to the `SMTP` constructor, use `functools.partial()` or write your own wrapper function. You might also want to add a signal handler so that the loop can be stopped, say when you hit control-C.

It’s probably easier to use a threaded controller which runs the SMTP server in a separate thread with a dedicated event loop. The controller provides useful and reliable `start` and `stop` semantics so that the foreground thread doesn’t block. Among other use cases, this makes it convenient to spin up an SMTP server for unit tests.

In both cases, you need to pass a handler to the `SMTP` constructor. Handlers respond to events that you care about during the SMTP dialog.

Important

Consider running the controller in a separate Python process (e.g., using the `multiprocessing` module) if you don’t want your main Python process to be blocked when aiosmtpd is handling extra-large emails.

## Using the controller

### TCP-based Server

The `Controller` class creates a TCP-based server, listening on an Internet endpoint (i.e., `ip_address:port` pair).

Say you want to receive email for `example.com` and print incoming mail data to the console. Start by implementing a handler as follows:

    >>> import asyncio
    >>> class ExampleHandler:
    ...     async def handle_RCPT(self, server, session, envelope, address, rcpt_options):
    ...         if not address.endswith('@example.com'):
    ...             return '550 not relaying to that domain'
    ...         envelope.rcpt_tos.append(address)
    ...         return '250 OK'
    ...
    ...     async def handle_DATA(self, server, session, envelope):
    ...         print('Message from %s' % envelope.mail_from)
    ...         print('Message for %s' % envelope.rcpt_tos)
    ...         print('Message data:\n')
    ...         for ln in envelope.content.decode('utf8', errors='replace').splitlines():
    ...             print(f'> {ln}'.strip())
    ...         print()
    ...         print('End of message')
    ...         return '250 Message accepted for delivery'

Pass an instance of your `ExampleHandler` class to the `Controller`, and then start it:

    >>> from aiosmtpd.controller import Controller
    >>> controller = Controller(ExampleHandler())
    >>> controller.start()

The SMTP thread might run into errors during its setup phase; to catch this the main thread will timeout when waiting for the SMTP server to become ready. By default the timeout is set to 1 second but can be changed either by using the `AIOSMTPD_CONTROLLER_TIMEOUT` environment variable or by passing a different `ready_timeout` duration to the Controller’s constructor.

Connect to the server and send a message, which then gets printed by `ExampleHandler`:

    >>> from smtplib import SMTP as Client
    >>> client = Client(controller.hostname, controller.port)
    >>> r = client.sendmail('a@example.com', ['b@example.com'], """\
    ... From: Anne Person <anne@example.com>
    ... To: Bart Person <bart@example.com>
    ... Subject: A test
    ... Message-ID: <ant>
    ...
    ... Hi Bart, this is Anne.
    ... """)
    Message from a@example.com
    Message for ['b@example.com']
    Message data:

    > From: Anne Person <anne@example.com>
    > To: Bart Person <bart@example.com>
    > Subject: A test
    > Message-ID: <ant>
    >
    > Hi Bart, this is Anne.

    End of message

You’ll notice that at the end of the `DATA` command, your handler’s `handle_DATA()` method was called. The sender, recipients, and message contents were taken from the envelope, and printed at the console. The handler methods also returns a successful status message.

The `ExampleHandler` class also implements a `handle_RCPT()` method. This gets called after the `RCPT TO` command is sanity checked. The method ensures that all recipients are local to the `@example.com` domain, returning an error status if not. It is the handler’s responsibility to add valid recipients to the `rcpt_tos` attribute of the envelope and to return a successful status.

Thus, if we try to send a message to a recipient not inside `example.com`, it is rejected:

    >>> client.sendmail('aperson@example.com', ['cperson@example.net'], """\
    ... From: Anne Person <anne@example.com>
    ... To: Chris Person <chris@example.net>
    ... Subject: Another test
    ... Message-ID: <another>
    ...
    ... Hi Chris, this is Anne.
    ... """)
    Traceback (most recent call last):
    ...
    smtplib.SMTPRecipientsRefused: {'cperson@example.net': (550, b'not relaying to that domain')}

When you’re done with the SMTP server, stop it via the controller.

    >>> controller.stop()

The server is guaranteed to be stopped.

    >>> client.connect(controller.hostname, controller.port)
    Traceback (most recent call last):
    ...
    ConnectionRefusedError: ...

There are a number of built-in handler classes that you can use to do some common tasks, and it’s easy to write your own handler. For a full overview of the methods that handler classes may implement, see the section on handler hooks.

### Unix Socket-based Server

The `UnixSocketController` class creates a server listening to a Unix Socket (i.e., a special file that can act as a ‘pipe’ for interprocess communication).

Usage is identical with the example described in the TCP-based Server section above, with some differences:

Rather than specifying a hostname:port to listen on, you specify the Socket’s filepath:

    >>> from aiosmtpd.controller import UnixSocketController
    >>> from aiosmtpd.handlers import Sink
    >>> controller = UnixSocketController(Sink(), unix_socket="smtp_socket~")
    >>> controller.start()

Warning

Do not exceed the Operating System limit for the length of the socket file path. On Linux, the limit is 108 characters. On BSD OSes, it’s 104 characters.

Rather than connecting to IP:port, you connect to the Socket file. Python’s `smtplib.SMTP` class sadly cannot connect to a Unix Socket, so we need to handle it on our own here:

    >>> import socket
    >>> sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
    >>> sock.connect("smtp_socket~")
    >>> sock.recv(1024)
    b'220 ...'

Try sending something, don’t forget to end with `"\r\n"`:

    >>> sock.send(b"HELO example.org\r\n")
    18
    >>> sock.recv(1024)
    b'250 ...'

And close everything when done:

    >>> sock.send(b"QUIT\r\n")
    6
    >>> sock.recv(1024)
    b'221 Bye...'
    >>> sock.close()
    >>> controller.stop()

### Unthreaded Controllers

In addition to the threaded controllers described above, `aiosmtpd` also provides the following UNthreaded controllers:

- `UnthreadedController` – the unthreaded version of `Controller`

- `UnixSocketUnthreadedController` – the unthreaded version of `UnixSocketController`

These classes are considered advanced classes, because you’ll have to manage the event loop yourself.

For example, to start an unthreaded controller, you’ll have to do something similar to this:

    >>> import asyncio
    >>> loop = asyncio.new_event_loop()
    >>> asyncio.set_event_loop(loop)
    >>> from aiosmtpd.controller import UnthreadedController
    >>> from aiosmtpd.handlers import Sink
    >>> controller = UnthreadedController(Sink(), loop=loop)
    >>> controller.begin()

Note that unlike the threaded counterparts, the method used to start the controller is named `begin()`. And unlike the method in the threaded version, `begin()` does NOT start the asyncio event loop; you’ll have to start it yourself.

For the purposes of trying this, let’s create a thread and have it run the asyncio event loop; we’ll also schedule an autostop so it won’t hang:

    >>> def runner():
    ...     # Set the delay to something long enough so you have time
    ...     # to do some testing
    ...     loop.call_later(3.0, loop.stop)
    ...     loop.run_forever()
    >>> import threading
    >>> thread = threading.Thread(target=runner)
    >>> thread.daemon = True
    >>> thread.start()
    >>> import time
    >>> time.sleep(0.1)  # Allow the loop to begin

At this point in time, the server would be listening:

    >>> from smtplib import SMTP as Client
    >>> client = Client(controller.hostname, controller.port)
    >>> client.helo("example.com")
    (250, ...)
    >>> client.quit()
    (221, b'Bye')

The complex thing will be to end it; that is why we’re marking these classes as “advanced”.

For our example here, since we have created an “autostop loop”, all we have to do is wait for the runner thread to end:

    >>> thread.join()
    >>> loop.is_running()
    False

We still need to do some cleanup to fully release the bound port. Since the loop has ended, we can simply call the `end()` method:

    >>> controller.end()

If you want to end the controller but keep the loop running, you’ll have to do it like this:

    loop.call_soon_threadsafe(controller.end)
    # If you want to ensure that controller has stopped, you can wait() here:
    controller.ended.wait(10.0)  # Optional

You must remember to cleanup the canceled tasks yourself. We have provided a convenience method, `cancel_tasks()`:

    # Will also stop the loop!
    loop.call_soon_threadsafe(controller.cancel_tasks)

(If you invoke `cancel_tasks` with the parameter `stop_loop=False`, then loop will NOT be stopped. That is a much too-advanced topic and we will not discuss it further in this documentation.)

The Unix Socket variant, `UnixSocketUnthreadedController`, works in the same way. The difference is only in how to access the server, i.e., through a Unix Socket instead of TCP/IP. We’ll leave out the details for you to figure it out yourself.

## Enabling SMTPUTF8

It’s very common to want to enable the `SMTPUTF8` ESMTP option, therefore this is the default for the `Controller` constructor. For backward compatibility reasons, this is not the default for the `SMTP` class though. If you want to disable this in the `Controller`, you can pass this argument into the constructor:

    >>> from aiosmtpd.handlers import Sink
    >>> controller = Controller(Sink(), enable_SMTPUTF8=False)
    >>> controller.start()
    >>>
    >>> client = Client(controller.hostname, controller.port)
    >>> code, message = client.ehlo('me')
    >>> code
    250

The EHLO response does not include the `SMTPUTF8` ESMTP option.

    >>> lines = message.decode('utf-8').splitlines()
    >>> # Don't print the server host name line, since that's variable.
    >>> for line in lines[1:]:
    ...     print(line)
    SIZE 33554432
    8BITMIME
    HELP

Stop the controller if we’re done experimenting:

    >>> controller.stop()

## Controller API

aiosmtpd.controller.DEFAULT_READY_TIMEOUT: float = 5.0

aiosmtpd.controller.get_localhost()
  Returns:

    The numeric address of the loopback interface; `"::1"` if IPv6 is supported, `"127.0.0.1"` if IPv6 is not supported.

  Return type:

    Literal[“::1”, “127.0.0.1”]

class aiosmtpd.controller.IP6_IS
  NO: set[int]

    Contains constants from `errno` that will be raised by `socket.socket.bind()` if IPv6 is NOT available on the system.

  YES: set[int]

    Contains constants from `errno` that will be raised by `socket.socket.bind()` if IPv6 IS available on the system.

  Note

  You can customize the contents of these attributes by adding/removing from them, in case the behavior does not align with your expectations and you cannot wait for a patch to be merged.

class aiosmtpd.controller.BaseController(handler, loop=None, *, ssl_context=None, server_hostname=None, server_kwargs=None, **SMTP_parameters)

  This Abstract Base Class defines parameters, attributes, and methods common between all concrete controller classes.

  Parameters:

    - handler – Handler object

    - loop (asyncio.AbstractEventLoop) – The asyncio event loop in which the server will run. If not given, `asyncio.new_event_loop()` will be called to create the event loop.

    - ssl_context (ssl.SSLContext) – SSL Context to wrap the socket in. Will be passed-through to `create_server()` method

    - server_hostname (Optional[str]) – Server’s hostname, will be passed-through as `hostname` parameter of `SMTP`

    - server_kwargs (dict) – (DEPRECATED) A dict that will be passed-through as keyword arguments of `SMTP`. This is DEPRECATED; please use `**SMTP_parameters` instead.

    - SMTP_parameters – Optional keyword arguments that will be passed-through as keyword arguments of `SMTP`

  Attributes
  handler

    The instance of the event handler passed to the constructor.

  loop

    The event loop being used.

  server

    This is the server instance returned by `_create_server()` after the server has started.

    You can retrieve the `socket` objects the server is listening on from the `server.sockets` attribute.

  smtpd: aiosmtpd.smtp.SMTP

    The server instance (of class SMTP) created by `factory()` after the controller is started.

  Methods
  factory() → aiosmtpd.smtp.SMTP

    You can override this method to create custom instances of the `SMTP` class being controlled.

    By default, this creates an `SMTP` instance, passing in your handler and setting flags from the `**SMTP_Parameters` parameter.

    Examples of why you would want to override this method include creating an LMTP server instance instead of the standard `SMTP` server.

  cancel_tasks(stop_loop=True)
    Parameters:

      stop_loop (bool) – If `True`, stops the loop before canceling tasks.

    This is a convenience class that will stop the loop & cancel all asyncio tasks for you.

class aiosmtpd.controller.Controller(handler, hostname=None, port=8025, loop=None, *, ready_timeout=DEFAULT_READY_TIMEOUT, ssl_context=None, server_hostname=None, server_kwargs=None, **SMTP_parameters)

  A concrete subclass of `BaseController` that provides a threaded, INET listener.

  Parameters:

    - hostname (Optional[str]) – Will be given to the event loop’s `create_server()` method as the `host` parameter, with a slight processing (see below)

    - port (int) – Will be passed-through to `create_server()` method

    - ready_timeout (float) – How long to wait until server starts. The `AIOSMTPD_CONTROLLER_TIMEOUT` takes precedence over this parameter. See `ready_timeout` for more information.

  Other parameters are defined in the `BaseController` class.

  The `hostname` parameter will be passed to the event loop’s `create_server()` method as the `host` parameter, except `None` (default) will be translated to `::1`.

  > - To bind dual-stack locally, use `localhost`.

  > - To bind dual-stack on all interfaces, use `""` (empty string).

  Important

  The `hostname` parameter does NOT get passed through to the SMTP instance; if you want to give the SMTP instance a custom hostname (e.g., for use in HELO/EHLO greeting), you must pass it through the `server_hostname` parameter.

  Explicitly defined SMTP keyword arguments will override keyword arguments of the same names defined in the (deprecated) `server_kwargs` argument.

      >>> from aiosmtpd.controller import Controller
      >>> from aiosmtpd.handlers import Sink
      >>> controller = Controller(
      ...     Sink(), timeout=200, server_kwargs=dict(timeout=400)
      ... )
      >>> controller.SMTP_kwargs["timeout"]
      200

  Finally, setting the `ssl_context` parameter will switch the protocol to `SMTPS` mode, implying unconditional encryption of the connection, and preventing the use of the `STARTTLS` mechanism.

  Actual behavior depends on the subclass’s implementation.

  Attributes

  In addition to those provided by `BaseController`, this class provides the following:

  hostname: str
  port: int

    The values of the hostname and port arguments.

  ready_timeout: float

    The timeout value used to wait for the server to start.

    This will either be the value of the `AIOSMTPD_CONTROLLER_TIMEOUT` environment variable (converted to float), or the `ready_timeout` parameter.

    Setting this to a high value will NOT slow down controller startup, because it’s a timeout limit rather than a sleep delay. However, you may want to reduce the default value to something ‘just enough’ so you don’t have to wait too long for an exception, if problem arises.

    If this timeout is breached, a `TimeoutError` exception will be raised.

  Methods

  In addition to those provided by `BaseController`, this class provides the following:

  start() → None
    Raises:

      - TimeoutError – if the server takes too long to get ready, exceeding the `ready_timeout` parameter.

      - RuntimeError – if an unrecognized & unhandled error happened, resulting in non-creation of a server object (`smtpd` remains `None`)

    Start the server in the subthread. The subthread is always a `daemon thread` (i.e., we always set `thread.daemon=True`).

    Exceptions can be raised if the server does not start within `ready_timeout` seconds, or if any other exception occurs in `factory()` while creating the server.

    Important

    If `start()` raises an Exception, cleanup is not performed automatically, to support deep inspection post-exception (if you wish to do so.) Cleanup must still be performed manually by calling `stop()`

    For example:

        # Assume SomeController is a concrete subclass of BaseThreadedController
        controller = SomeController(handler)
        try:
            controller.start()
        except ...:
            ... exception handling and/or inspection ...
        finally:
            controller.stop()

  stop(no_assert=False) → None
    Parameters:

      no_assert (bool) – If `True`, skip the assertion step so an `AssertionError` will not be raised if thread had not been started successfully.

    Raises:

      AssertionError – if this method is called before `start()` is called successfully AND `no_assert=False`

    Stop the server and the event loop, and cancel all tasks via `cancel_tasks()`.

class aiosmtpd.controller.UnixSocketController(handler, unix_socket, loop=None, *, ready_timeout=DEFAULT_READY_TIMEOUT, ssl_context=None, server_hostname=None, **SMTP_parameters)

  A concrete subclass of `BaseController` that provides a threaded, Unix Socket listener.

  Parameters:

    unix_socket (Union[str, pathlib.Path]) – Socket file, will be passed-through to `asyncio.loop.create_unix_server()`

  For the other parameters, see the description under `Controller`

  Attributes
  unix_socket: str

    The stringified version of the `unix_socket` parameter

  Other attributes (except `hostname` and `port`) are identical to `Controller` and thus are not repeated nor explained here.

  Methods

  All methods are identical to `Controller` and thus are not repeated nor explained here.

class aiosmtpd.controller.UnthreadedController(handler, hostname=None, port=8025, loop=None, *, ssl_context=None, server_hostname=None, server_kwargs=None, **SMTP_parameters)

  Added in version 1.5.0.

  A concrete subclass of `BaseController` that provides an UNthreaded, INET listener.

  Parameters are identical to the `Controller` class.

  Attributes

  Attributes are identical to the `Controller` class with one addition:

  ended: threading.Event

    An `Event` that can be `.wait()`-ed when ending the controller. Please see the Unthreaded Controllers section for more info.

  Methods

  In addition to those provided by `BaseController`, this class provides the following:

  begin()

    Initializes the server task and insert it into the asyncio event loop.

    Note

    The SMTP class itself will only be initialized upon first connection to the server task.

  async finalize()

    Perform orderly closing of the server listener. If you need to close the server from a non-async function, you can use the `end()` method instead.

    Upon completion of this method, the `ended` attribute will be `set()`.

  end()

    This is a convenience method that will asynchronously invoke the `finalize()` method. This method non-async, and thus is callable from non-async functions.

    Note

    If the asyncio event loop has been stopped, then it is safe to invoke this method directly. Otherwise, it is recommended to invoke this method using the `call_soon_threadsafe()` method.

class aiosmtpd.controller.UnixSocketUnthreadedController(handler, unix_socket, loop=None, *, ssl_context=None, server_hostname=None, server_kwargs=None, **SMTP_parameters)

  Added in version 1.5.0.

  A concrete subclass of `BaseController` that provides an UNthreaded, Unix Socket listener.

  Parameters are identical to the `UnixSocketController` class.

  Attributes

  Attributes are identical to the `UnixSocketController` class, with the following addition:

  ended: threading.Event

    An `Event` that can be `.wait()`-ed when ending the controller. Please see the Unthreaded Controllers section for more info.

  Methods

  Methods are identical to the `UnthreadedController` class.