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, ident=None, tls_context=None, require_starttls=False, timeout=300, auth_required=False, auth_require_tls=True, auth_exclude_mechanism=None, auth_callback=None, 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 first part of 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.

ident is the second part of the string returned in the 220 greeting response that identifies the software name and version of the SMTP server to the client. If not given, a default Python SMTP ident is used.

tls_context and require_starttls. The STARTTLS option of ESMTP (and LMTP), defined in RFC 3207, provides for secure connections to the server. For this option to be available, tls_context must be supplied, and require_starttls should be True. See Enabling STARTTLS for a more in depth discussion on enabling STARTTLS.

timeout is the number of seconds to wait between valid SMTP commands. After this time the connection will be closed by the server. The default is 300 seconds, as per RFC 2821.

auth_required specifies whether SMTP Authentication is mandatory or not for the session. This impacts some SMTP commands such as HELP, MAIL FROM, RCPT TO, and others.

auth_require_tls specifies whether STARTTLS must be used before AUTH exchange or not. If you set this to False then AUTH exchange can be done outside a TLS context, but the class will warn you of security considerations. Please note that require_starttls takes precedence over this setting.

auth_exclude_mechanism is an Iterable[str] that specifies SMTP AUTH mechanisms to NOT use.

auth_callback is a function that accepts three arguments: mechanism: str, login: bytes, and password: bytes. Based on these args, the function must return a bool that indicates whether the client’s authentication attempt is accepted/successful or not.

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 flag that indicates whether authentication had succeeded.


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.


To enable RFC 3207 STARTTLS, you must supply the tls_context argument to the SMTP class. tls_context is created with the ssl.create_default_context() call from the ssl module, as follows:

context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)

The context must be initialized with a server certificate, private key, and/or intermediate CA certificate chain with the ssl.SSLContext.load_cert_chain() method. This can be done with separate files, or an all in one file. Files must be in PEM format.

For example, if you wanted to use a self-signed certification for localhost, which is easy to create but doesn’t provide much security, you could use the openssl(1) command like so:

$ openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes -subj '/CN=localhost'

and then in Python:

context = ssl.create_default_context(ssl.Purpose.CLIENT_AUTH)
context.load_cert_chain('cert.pem', 'key.pem')

Now pass the context object to the tls_context argument in the SMTP constructor.

Note that a number of exceptions can be generated by these methods, and by SSL connections, which you must be prepared to handle. Additional documentation is available in Python’s ssl module, and should be reviewed before use; in particular if client authentication and/or advanced error handling is desired.

If require_starttls is True, a TLS session must be initiated for the server to respond to any commands other than EHLO/LHLO, NOOP, QUIT, and STARTTLS.

If require_starttls is False (the default), use of TLS is not required; the client may upgrade the connection to TLS, or may use any supported command over an insecure connection.

If tls_context is not supplied, the STARTTLS option will not be advertised, and the STARTTLS command will not be accepted. require_starttls is meaningless in this case, and should be set to False.