The SMTP class

At the heart of this module is the SMTP class. This class implements the RFC 5321 Simple Mail Transport Protocol. Often you won’t run an SMTP instance directly, but instead will use a controller instance to run the server in a subthread.

>>> from aiosmtpd.controller import Controller

The SMTP class is itself a subclass of StreamReaderProtocol.


While behavior for common SMTP commands can be specified using handlers, more complex specializations such as adding custom SMTP commands require subclassing the SMTP class.

For example, let’s say you wanted to add a new SMTP command called PING. All methods implementing SMTP commands are prefixed with smtp_; they must also be coroutines. Here’s how you could implement this use case:

>>> import asyncio
>>> from aiosmtpd.smtp import SMTP as Server, syntax
>>> class MyServer(Server):
...     @syntax('PING [ignored]')
...     async def smtp_PING(self, arg):
...         await self.push('259 Pong')

Now let’s run this server in a controller:

>>> from aiosmtpd.handlers import Sink
>>> class MyController(Controller):
...     def factory(self):
...         return MyServer(self.handler)

>>> controller = MyController(Sink())
>>> controller.start()

We can now connect to this server with an SMTP client.

>>> from smtplib import SMTP as Client
>>> client = Client(controller.hostname, controller.port)

Let’s ping the server. Since the PING command isn’t an official SMTP command, we have to use the lower level interface to talk to it.

>>> code, message = client.docmd('PING')
>>> code
>>> message

Because we prefixed the smtp_PING() method with the @syntax() decorator, the command shows up in the HELP output.

>>> print('utf-8'))

And we can get more detailed help on the new command.

>>> print('PING').decode('utf-8'))
Syntax: PING [ignored]

Server hooks


These methods are deprecated. See handler hooks instead.

The SMTP server class also implements some hooks which your subclass can override to provide additional responses.

This hook makes it possible for subclasses to return additional EHLO responses. This method, called asynchronously and taking no arguments, can do whatever it wants, including (most commonly) pushing new 250-<command> responses to the client. This hook is called just before the standard 250 HELP which ends the EHLO response from the server.
This hook makes it possible to return additional RSET responses. This method, called asynchronously and taking no arguments, is called just before the standard 250 OK which ends the RSET response from the server.


class SMTP(handler, *, data_size_limit=33554432, enable_SMTPUTF8=False, decode_data=False, hostname=None, tls_context=None, require_starttls=False, loop=None)

handler is an instance of a handler class.

data_size_limit is the limit in number of bytes that is accepted for client SMTP commands. It is returned to ESMTP clients in the 250-SIZE response. The default is 33554432.

enable_SMTPUTF8 is a flag that when True causes the ESMTP SMTPUTF8 option to be returned to the client, and allows for UTF-8 content to be accepted. The default is False.

decode_data is a flag that when True, attempts to decode byte content in the DATA command, assigning the string value to the envelope’s content attribute. The default is False.

hostname is the string returned in the 220 greeting response given to clients when they first connect to the server. If not given, the system’s fully-qualified domain name is used.

tls_context and require_starttls are related to the ESMTP STARTTLS command for secure connections to the server, based on RFC 3207. tls_context is used as the SSL protocol context, and there is no default. tls_context must be given and require_starttls must be True for STARTTLS to be supported.

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


The handler instance passed into the constructor.


The value of the data_size_limit argument passed into the constructor.


The value of the enable_SMTPUTF8 argument passed into the constructor.


The 220 greeting hostname. This will either be the value of the hostname argument passed into the constructor, or the system’s fully qualified host name.


The value of the tls_context argument passed into the constructor.


True if both the tls_context argument to the constructor was given and the require_starttls flag was True.


The active session object, if there is one, otherwise None.


The active envelope object, if there is one, otherwise None.


The active asyncio transport if there is one, otherwise None.


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


A method subclasses can override to return custom Session instances.


A method subclasses can override to return custom Envelope instances.


The method that subclasses and handlers should use to return statuses to SMTP clients. This is a coroutine. status can be a bytes object, but for convenience it is more likely to be a string. If it’s a string, it must be ASCII, unless enable_SMTPUTF8 is True in which case it will be encoded as UTF-8.


Coroutine methods implementing the SMTP protocol commands. For example, smtp_HELO() implements the SMTP HELO command. Subclasses can override these, or add new command methods to implement custom extensions to the SMTP protocol. arg is the rest of the SMTP command given by the client, or None if nothing but the command was given.