The SMTP Module

At the heart of this module is the SMTP class in the aiosmtpd.smtp module. 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]

Don’t forget to stop() the controller when you’re done.

>>> controller.stop()

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.

Deprecated since version 1.2.


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.

Deprecated since version 1.2.


aiosmtpd.smtp.AuthenticatorType = Callable[[SMTP, Session, Envelope, str, Any], AuthResult]

actual_name (str) – Name of the AUTH Mechanism implemented by the method. See AUTH Mechanism Hooks for more info.

This decorator specifies the actual name of the AUTH Mechanism implemented by the method being decorated, regardless of the method’s name.


The decorated method’s name MUST still start with auth_

class aiosmtpd.smtp.AuthResult

Contains the result of the Authentication Procedure.

For more info, please see AuthResult

class aiosmtpd.smtp.LoginPassword(login: bytes, password: bytes)

A subclass of typing.NamedTuple that holds the Authentication Data for the built-in LOGIN and PLAIN AUTH Mechanisms.

It is to be used for Authentication purposes by Authenticator()

For more information, please refer to the Authentication System page.

class aiosmtpd.smtp.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, authenticator=None, command_call_limit=None, proxy_protocol_timeout=None, loop=None)


An instance of a handler class that optionally can implement Handler Hooks.

data_size_limit: int = 33554432

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

enable_SMTPUTF8: bool = False

When True, causes the ESMTP SMTPUTF8 option to be returned to the client, and allows for UTF-8 content to be accepted, as defined in RFC 6531.

decode_data: bool = False

When True, attempts to decode byte content in the DATA command, assigning the string value to the envelope’s content attribute.

hostname: str | None = None

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: str | None = None

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: ssl.SSLContext | None = None

An instance of ssl.SSLContext. Providing this will enable support for STARTTLS ESMTP/LMTP option as defined in RFC 3207.

See Enabling STARTTLS for a more in-depth discussion on enabling STARTTLS.

require_starttls: bool = False

If set to True, then client must send STARTTLS before “restricted” ESMTP commands can be issued.

“Restricted” ESMTP commands are all commands not in the set {"NOOP", "EHLO", "STARTTLS", "QUIT"}

timeout: int | float = 300

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: bool = False

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: bool = True

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.

Has no effect if require_starttls is True.

auth_exclude_mechanism: Iterable[str] | None = None

Specifies which AUTH mechanisms to NOT use.

This is the only way to completely disable the built-in AUTH mechanisms.

See Authentication System for a more in-depth discussion on AUTH mechanisms.

Added in version 1.2.2.

auth_callback: Callable[[str, bytes, bytes], bool] = login_always_fail

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.

Deprecated since version 1.3: Use authenticator instead. This parameter will be removed in version 2.0.

authenticator: aiosmtpd.smtp.AuthenticatorType = None

A function whose signature is identical to aiosmtpd.smtp.AuthenticatorType.

See Authenticator() for more information.

Added in version 1.3.

command_call_limit: int | Dict[str, int] | None = None

If not None sets the maximum time a certain SMTP command can be invoked. This is to prevent DoS due to malicious client connecting and never disconnecting, due to continual sending of SMTP commands to prevent timeout.

The handling differs based on the type:

If command_call_limit is of type int, then the value is the call limit for ALL SMTP commands.

If command_call_limit is of type dict, it must be a Dict[str, int] (the type of the values will be enforced). The keys will be the SMTP Command to set the limit for, the values will be the call limit per SMTP Command.

A special key of "*" is used to set the ‘default’ call limit for commands not explicitly declared in command_call_limit. If "*" is not given, then the ‘default’ call limit will be set to aiosmtpd.smtp.CALL_LIMIT_DEFAULT

Other types – or a Dict whose any value is not an int – will raise a TypeError exception.


# All commands have a limit of 10 calls
SMTP(..., command_call_limit=10)

# Commands RCPT and NOOP have their own limits; others have an implicit limit
SMTP(..., command_call_limit={"RCPT": 30, "NOOP": 5})

# Commands RCPT and NOOP have their own limits; others set to 3
SMTP(..., command_call_limit={"RCPT": 20, "NOOP": 10, "*": 3})

If not given (or set to None), then command call limit will not be enforced. This will change in version 2.0.

Added in version 1.2.3.

proxy_protocol_timeout: int | float | None = None

If given (not None), activates support for PROXY Protocol.

Please read the PROXY Protocol Support documentation for a more in-depth explanation.

If not given (or None), disables support for PROXY Protocol.


When PROXY protocol support is activated, SMTP’s behavior changes: It no longer immediately sends 220 greeting upon client connection, but instead it will wait for client to first send the PROXY protocol header.

This is in accordance to the PROXY Protocol standard.

Added in version 1.4.


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

Attributes & Methods

The maximum line length, in octets (not characters; one UTF-8 character may result in more than one octet). Defaults to 1001 in compliance with RFC 5321 §


This sets the stream limit of asyncio.StreamReader.readuntil(), thus impacting how the method works. In previous versions of aiosmtpd, the limit is not set. To return to the behavior of the previous versions, set line_length_limit to 2**16 before instantiating the SMTP class.


The maximum lengh (in octets) of the local part of email addresses.

RFC 5321 § specifies a maximum length of 64 octets, but this requirement is flexible and can be relaxed at the server’s discretion (see §

Setting this to 0 (the default) disables this limit completely.


A str containing the base64-encoded challenge to be sent as the first challenge in the AUTH LOGIN mechanism.


A str containing the base64-encoded challenge to be sent as the second challenge in the AUTH LOGIN mechanism.


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.

async push(status)

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.

async challenge_auth(challenge, encode_to_b64=True, log_client_response=False) _Missing | bytes
  • challenge (AnyStr) – The SMTP AUTH challenge to send to the client. May be in plaintext, may be in base64. Do NOT prefix with “334 “!

  • encode_to_b64 (bool) – If true, will perform base64-encoding before sending the challenge to the client.

  • log_client_response (bool) – If true, will perform logging of client response


Response from client (already base64-decoded) or MISSING (see description)

This method will return MISSING if either of these scenarios happen:

  • client aborted the AUTH procedure by sending b"*", or

  • client response to the challenge cannot be base64-decoded.


Setting log_client_response=True might cause leakage of sensitive information!



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.