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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


  • Postfix 2.10 and later
  • Python 3.5.3+ (see "Systems without Python 3.5+" below if you haven't one)
  • aiodns
  • aiohttp
  • pynetstring
  • PyYAML
  • (optional) uvloop

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 and FreeBSD 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-
  -l FILE, --logfile FILE
                        log file location (default: None)                     
  --disable-uvloop      do not use uvloop even if it is available (default:

Seamless restart/upgrade/reload and load balancing

By default mta-sts-daemon allows its multiple instances to share same port (on Linux/FreeBSD/Windows). Therefore, restart or upgrade of daemon can be performed seamlessly. Set of unit files for systemd in contrib/ directory implements "reload" by mean of running backup instance when main instance is getting restarted.

Also on Linux and FreeBSD, load distribited across all processes (with SO_REUSEPORT and SO_REUSEPORT_LB respectively).

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

SMTP client of your Postfix instance must be able to validate peer certificates. In order to achieve that, you have to ensure smtp_tls_CAfile or smtp_tls_CApath points to system CA bundle. Otherwise you'll get Unverified TLS connection even for peers with valid certificate, and delivery failures for MTA-STS-enabled destinations. Also note: even enabled tls_append_default_CA will not work alone if both smtp_tls_CAfile and smtp_tls_CApath are empty.

Once certificate validation is enabled and your Postfix log shows "Trusted TLS connection ... " for destinations with valid certificates signed by public CA, you may enable MTA-STS by adding following line to

smtp_tls_policy_maps = socketmap:inet:

If your configuration already has some TLS policy maps, just add MTA-STS socketmap to list of configured maps accordingly to smtp_tls_policy_maps syntax. TLS policy tables are searched in the specified order until a match is found, so you may have table with local overrides of TLS policy prior to MTA-STS socketmap. This may be useful for skipping network lookup for well-known destinations or relaxing security for broken destinations, announcing MTA-STS support.

Reload Postfix after reconfiguration.

Operability check

Assuming default MTA-STA daemon configuration. Following command:

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

should return something like:


Postfix log should show Verified TLS connection established to ... instead of Untrusted ... or 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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