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 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

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')
...         print(envelope.content.decode('utf8', errors='replace'))
...         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.

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
>>> controller.stop()

Controller API

class Controller(handler, loop=None, hostname=None, port=8025, *, ready_timeout=1.0, enable_SMTPUTF8=True, ssl_context=None, server_kwargs=None)

handler is an instance of a handler class.

loop is the asyncio event loop to use. If not given, asyncio.new_event_loop() is called to create the event loop.

hostname is passed to your loop’s AbstractEventLoop.create_server() method as the host parameter, except None (default) is translated to ‘::1’. To bind dual-stack locally, use ‘localhost’. To bind dual-stack on all interfaces, use ‘’.

port is passed directly to your loop’s AbstractEventLoop.create_server() method.

ready_timeout is float number of seconds that the controller will wait in Controller.start() for the subthread to start its server. You can also set the AIOSMTPD_CONTROLLER_TIMEOUT environment variable to a float number of seconds, which takes precedence over the ready_timeout argument value.

enable_SMTPUTF8 is a flag which is passed directly to the same named argument to the SMTP constructor. When True, the ESMTP SMTPUTF8 option is returned to the client in response to EHLO, and UTF-8 content is accepted.

ssl_context is an SSLContext that will be used by the loop’s server. It is passed directly to the AbstractEventLoop.create_server() method. Note that this implies unconditional encryption of the connection, and prevents use of the STARTTLS mechanism.

server_kwargs is a dict that will be passed through as keyword arguments to the server’s class during server creation in the Controller.factory() method.


The instance of the event handler passed to the constructor.


The event loop being used. This will either be the given loop argument, or the new event loop that was created.


The values of the hostname and port arguments.


The timeout value used to wait for the server to start. This will either be the float value converted from the AIOSMTPD_CONTROLLER_TIMEOUT environment variable, or the ready_timeout argument.


This is the server instance returned by AbstractEventLoop.create_server() after the server has started.


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 the ready_timeout, or if any other exception occurs in while creating the server.


Stop the server and the event loop, and cancel all tasks.


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 the enable_SMTPUTF8 flag. Examples of why you would want to override this method include creating an LMTP server instance instead, or passing in a different set of arguments to the SMTP constructor.