aiosmtpd - An asyncio based SMTP server

Project License on GitHub PyPI Package Supported Python Versions
GitHub Actions status Code Coverage Semmle/ quality Documentation Status


The Python standard library includes a basic SMTP server in the smtpd module, based on the old asynchronous libraries asyncore and asynchat. These modules are quite old and are definitely showing their age; asyncore and asynchat are difficult APIs to work with, understand, extend, and fix.

With the introduction of the asyncio module in Python 3.4, a much better way of doing asynchronous I/O is now available. It seems obvious that an asyncio-based version of the SMTP and related protocols are needed for Python 3. This project brings together several highly experienced Python developers collaborating on this reimplementation.

This package provides such an implementation of both the SMTP and LMTP protocols.

Full documentation is available on


You need at least Python 3.6 to use this library.

Supported Platforms

aiosmtpd has been tested on CPython and PyPy3.7 for the following platforms (in alphabetical order):

  • Cygwin (on Windows 10) [1]

  • FreeBSD 12 [2]

  • OpenSUSE Leap 15 [2]

  • Ubuntu 18.04

  • Ubuntu 20.04

  • Windows 10

    [1] Supported only with Cygwin-provided CPython versions
    [2] Supported only on the latest minor release

aiosmtpd probably can run on platforms not listed above, but we cannot provide support for unlisted platforms.


Install as usual with pip:

pip install aiosmtpd

If you receive an error message ModuleNotFoundError: No module named 'public', it likely means your setuptools is too old; try to upgrade setuptools to at least version 46.4.0 which had implemented a fix for this issue.

Project details

As of 2016-07-14, aiosmtpd has been put under the aio-libs umbrella project and moved to GitHub.

The best way to contact the developers is through the GitHub links above. You can also request help by submitting a question on StackOverflow.


You can install this package in a virtual environment like so:

$ python3 -m venv /path/to/venv
$ source /path/to/venv/bin/activate
$ python install

This will give you a command line script called aiosmtpd which implements the SMTP server. Use aiosmtpd --help for a quick reference.

You will also have access to the aiosmtpd library, which you can use as a testing environment for your SMTP clients. See the documentation links above for details.


You’ll need the tox tool to run the test suite for Python 3. Once you’ve got that, run:

$ tox

Individual tests can be run like this:

$ tox -- <testname>

where <testname> is the “node id” of the test case to run, as explained in the pytest documentation. The command above will run that one test case against all testenvs defined in tox.ini (see below).

If you want test to stop as soon as it hit a failure, use the -x/--exitfirst option:

$ tox -- -x

You can also add the -s/--capture=no option to show output, e.g.:

$ tox -e py36-nocov -- -s

and these options can be combined:

$ tox -e py36-nocov -- -x -s <testname>

(The -e parameter is explained in the next section about ‘testenvs’. In general, you’ll want to choose the nocov testenvs if you want to show output, so you can see which test is generating which output.)

The -x and -s options can be combined:

$ tox -e py36-nocov -- -x -s <testname>

Supported ‘testenvs’

In general, the -e parameter to tox specifies one (or more) testenv to run (separate using comma if more than one testenv). The following testenvs have been configured and tested:

  • {py36,py37,py38,py39,pypy3}-{nocov,cov,diffcov,profile}

    Specifies the interpreter to run and the kind of testing to perform.

    • nocov = no coverage testing. Tests will run verbosely.

    • cov = with coverage testing. Tests will run in brief mode (showing a single character per test run)

    • diffcov = with diff-coverage report (showing difference in coverage compared to previous commit). Tests will run in brief mode

    • profile = no coverage testing, but code profiling instead. This must be invoked manually using the -e parameter

    Note 1: As of 2021-02-23, only the {py36,py38}-{nocov,cov} combinations work on Cygwin.

    Note 2: It is also possible to use whatever Python version is used when invoking tox by using the py target, but you must explicitly include the type of testing you want. For example:

    $ tox -e "py-{nocov,cov,diffcov}"

    (Don’t forget the quotes if you want to use braces!)

    You might want to do this for CI platforms where the exact Python version is pre-prepared, such as Travis CI or GitHub Actions; this will definitely save some time during tox’s testenv prepping.

    For all testenv combinations except diffcov, bandit security check will also be run prior to running pytest.

  • qa

    Performs flake8 code style checking, and flake8-bugbear design checking.

    In addition, some tests to help ensure that aiosmtpd is releasable to PyPI are also run.

  • docs

    Builds HTML documentation and manpage using Sphinx. A pytest doctest will run prior to actual building of the documentation.

  • static

    Performs a static type checking using pytype.

    Note 1: Please ensure that all pytype dependencies have been installed before executing this testenv.

    Note 2: This testenv will be _SKIPPED_ on Windows, because pytype currently cannot run on Windows.

    Note 3: This testenv does NOT work on Cygwin.

Environment Variables


Due to how asyncio event loop works, some actions do not instantly get responded to. This is especially so on slower / overworked systems. In consideration of such situations, some test cases invoke a slight delay to let the event loop catch up.

Defaults to 0.1 and can be set to any float value you want.

Different Python Versions

The tox configuration files have been created to cater for more than one Python versions safely: If an interpreter is not found for a certain Python version, tox will skip that whole testenv.

However, with a little bit of effort, you can have multiple Python interpreter versions on your system by using pyenv. General steps:

  1. Install pyenv from

  2. Install tox-pyenv from

  3. Using pyenv, install the Python versions you want to test on

  4. Create a .python-version file in the root of the repo, listing the Python interpreter versions you want to make available to tox (see pyenv’s documentation about this file)

    Tip: The 1st line of .python-version indicates your preferred Python version which will be used to run tox.

  5. Invoke tox with the option --tox-pyenv-no-fallback (see tox-pyenv’s documentation about this option)

If you ever need to ‘reset’ your repo, you can use the utility like so:

$ python superclean

It is strongly recommended to NOT do superclean too often, though. Every time you invoke superclean, tox will have to recreate all its testenvs, and this will make testing much longer to finish.

superclean is typically only needed when you switch branches, or if you want to really ensure that artifacts from previous testing sessions won’t interfere with your next testing sessions.

For example, you want to force Sphinx to rebuild all documentation. Or, you’re sharing a repo between environments (say, PSCore and Cygwin) and the cached Python bytecode messes up execution (e.g., sharing the exact same directory between Windows PowerShell and Cygwin will cause problems as Python becomes confused about the locations of the source code).

Signing Keys

Starting version 1.3.1, files provided through PyPI or GitHub Releases will be signed using one of the following GPG Keys:




5D60 CE28 9CD7 C258


pepoluan at gmail period com


aiosmtpd is released under the Apache License version 2.0.

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