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.

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 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(''):
...             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('', [''], """\
... From: Anne Person <>
... To: Bart Person <>
... Subject: A test
... Message-ID: <ant>
... Hi Bart, this is Anne.
... """)
Message from
Message for ['']
Message data:

> From: Anne Person <>
> To: Bart Person <>
> 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 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, it is rejected:

>>> client.sendmail('', [''], """\
... From: Anne Person <>
... To: Chris Person <>
... Subject: Another test
... Message-ID: <another>
... Hi Chris, this is Anne.
... """)
Traceback (most recent call last):
smtplib.SMTPRecipientsRefused: {'': (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()


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\r\n")
>>> sock.recv(1024)
b'250 ...'

And close everything when done:

>>> sock.send(b"QUIT\r\n")
>>> 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.get_event_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.setDaemon(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("")
(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()

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:

# 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!

(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

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

Stop the controller if we’re done experimenting:

>>> controller.stop()

Controller API

aiosmtpd.controller.DEFAULT_READY_TIMEOUT: float = 5.0

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

Return type

Literal[“::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.


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.

  • 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


The instance of the event handler passed to the constructor.


The event loop being used.


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.


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.


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.


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).


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"]

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.


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.


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

  • 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.


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)
except ...:
    ... exception handling and/or inspection ...

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


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.


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

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.


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)

New in version 1.5.0.

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

Parameters are identical to the Controller class.


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.


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


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


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().


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


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)

New 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 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 are identical to the UnthreadedController class.