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Daemon which provides TLS client policy for Postfix via socketmap, according to domain MTA-STS policy

Project description


Daemon which provides TLS client policy for Postfix via socketmap, according to domain MTA-STS policy. Current support of RFC8461 is limited - daemon lacks some minor features:

  • Proactive policy fetch
  • Fetch error reporting
  • Fetch ratelimit


All dependency packages installed automatically if this package is installed via pip.


Method 1. System-wide install from PyPI (recommended for humans)


sudo python3 -m pip install postfix-mta-sts-resolver

Package scripts shall be available in standard executable locations upon completion.

Method 2. System-wide install from project source

Run in project directory:

sudo python3 -m pip install .

Package scripts shall be available in standard executable locations upon completion.

Method 3. Running from project directory without installation

Installing dependencies:

sudo python3 -m pip install -r requirements.txt

Now scripts can be run right in source directory.

Method 4. Install into virtualenv

See "Building virtualenv"

Common installation notes

See also contrib/ for RHEL/OEL/Centos notes.

See contrib/postfix-mta-sts.service for example of systemd unit file suitable to run daemon under systemd control.

pip user install

All pip invocations can be run with --user option of pip installer. In this case superuser privileges are not required and package(s) are getting installed into user home directory. Usually, script executables will appear in ~/.local/bin.


This package provides two executables available after installation in respective locations.


mta-sts-query is a command line tool which fetches and outputs domain MTA-STS policies. Intended to be used for debug purposes.


$ mta-sts-query --help
usage: mta-sts-query [-h] [-v {debug,info,warn,error,fatal}]
                     domain [known_version]

positional arguments:
  domain                domain to fetch MTA-STS policy from
  known_version         latest known version (default: None)

optional arguments:
  -h, --help            show this help message and exit
  -v {debug,info,warn,error,fatal}, --verbosity {debug,info,warn,error,fatal}
                        logging verbosity (default: warn)


mta-sts-daemon is a daemon which provides external TLS policy for Postfix SMTP client via socketmap interface.

You may find useful systemd unit file to run daemon in contrib/postfix-mta-sts.service.


$ mta-sts-daemon --help
usage: mta-sts-daemon [-h] [-v {debug,info,warn,error,fatal}] [-c FILE]

optional arguments:
  -h, --help            show this help message and exit
  -v {debug,info,warn,error,fatal}, --verbosity {debug,info,warn,error,fatal}
                        logging verbosity (default: info)
  -c FILE, --config FILE
                        config file location (default: /etc/postfix/mta-sts-
  --disable-uvloop      do not use uvloop even if it is available (default:

MTA-STS Daemon configuration

See example config in source code directory. Default config location is: /etc/postfix/mta-sts-daemon.yml, but it can be overriden with command line option -c FILE.

All options is self-explanatory, only exception is strict_testing option. If set to true, STS policy will be enforced even if domain announces testing MTA-STS mode. Useful for premature incorporation of MTA-STS against domains hesistating to go enforce. Please use with caution.

Postfix configuration

Add line like

smtp_tls_policy_maps = socketmap:inet:

into your config and reload Postfix.

Operability check

Assuming default configuration. Following command:

/usr/sbin/postmap -q socketmap:inet:

should return something like:


Postfix log should show Verified TLS connection established to ... instead of Trusted TLS connection established to ... when mail is getting sent to MTA-STS-enabled domain.

Special cases of deployment

Systems without Python 3.5+

Some people may find convenient to install latest python from source into /opt directory. This way you can have separate python installation not interferring with system packages by any means. Download latest python source from, unpack and run in unpacked source directory:

./configure --prefix=/opt --enable-optimizations && make -j $[ $(nproc) + 1 ] && make test && sudo make install

Python binaries will be available in /opt/bin, including pip3. You may install postfix-mta-sts-resolver using /opt/bin/pip3 without interference with any system packages:

sudo /opt/bin/pip3 install postfix-mta-sts-resolver

Executable files of postfix-mta-sts-resolver will be available in /opt/bin/mta-sts-query and /opt/bin/mta-sts-daemon

Building virtualenv

Run make in project directory in order to build virtualenv. As result of it, new directory venv shall appear. venv contains interpreter and all required dependencies, i.e. encloses package with depencencies in separate environment. It is possible to specify alternative path where virtualenv directory shall be placed. Specify VENV variable for make command. Example:

make VENV=~/postfix-mta-sts-resolver

Such virtual environment can be moved to another machine of similar type (as far python interpreter is compatible with new environment). If virtualenv is placed into same location on new machine, application can be runned this way:


Otherwise, some hacks required. First option - explicitly call virtualenv interpreter:

venv/bin/python venv/bin/mta-sts-daemon

Second option - specify new path in shebang of scripts installed in virtualenv. It is recommended to build virtualenv at same location which app shall occupy on target system.


Inspired by this forum thread.

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