The usual python misery.

[jult]

Gosh how I detest python. Why is there rarely, if at all, any proper documentation on python projects out there?
Fails at even the first inkling of an install;

root@server:~## python3.7 -m pip install -U pipenv
-bash: python3.7: command not found

root@server:~# python3.11.2 -m pip install -U pipenv
-bash: python3.11.2: command not found

root@server:~# python3 -m pip install -U pipenv
error: externally-managed-environment

× This environment is externally managed
╰─> To install Python packages system-wide, try apt install
    python3-xyz, where xyz is the package you are trying to
    install.

    If you wish to install a non-Debian-packaged Python package,
    create a virtual environment using python3 -m venv path/to/venv.
    Then use path/to/venv/bin/python and path/to/venv/bin/pip. Make
    sure you have python3-full installed.

    If you wish to install a non-Debian packaged Python application,
    it may be easiest to use pipx install xyz, which will manage a
    virtual environment for you. Make sure you have pipx installed.

    See /usr/share/doc/python3.11/README.venv for more information.

note: If you believe this is a mistake, please contact your Python installation or OS distribution provider. You can override this, at the risk of breaking your Python installation or OS, by passing --break-system-packages.
hint: See PEP 668 for the detailed specification.

Then installed it using apt install pipenv.
Second hurdle with your 'instructions':

root@server:~# gpasswd -a mailparser syslog adm postfix
Usage: gpasswd [option] GROUP

Options:
  -a, --add USER                add USER to GROUP
  -d, --delete USER             remove USER from GROUP
  -h, --help                    display this help message and exit
  -Q, --root CHROOT_DIR         directory to chroot into
  -r, --remove-password         remove the GROUP's password
  -R, --restrict                restrict access to GROUP to its members
  -M, --members USER,...        set the list of members of GROUP
  -A, --administrators ADMIN,...
                                set the list of administrators for GROUP
Except for the -A and -M options, the options cannot be combined.

Seems there is no adm or syslog group or user on a default debian 12 system. So had to move those out.
Third hurdble:

mailparser@server:~/postfix-parser$ pipenv install
Warning: Python 3.8 was not found on your system...
Neither 'pyenv' nor 'asdf' could be found to install Python.
You can specify specific versions of Python with:
$ pipenv --python path/to/python

and then I tried:

root@server:~# whereis python
python:

So, your Install is pretty much moot there; Had to change the Pipfile entry where it requires Python 3.8 to 3.11.2 and now it finally runs!

Also

# Add a crontab entry to run the parse/import script every minute or so
# You should use a file lock utility such as `flock` (included by default on Ubuntu) or `lckdo` to prevent the
# cron overlapping if there's a lot to parse.

crontab -e
# *  *   *   *   *    flock /tmp/lck_mailparser /home/mailparser/postfix-parser/run.sh cron

I have no idea what you're trying to say here, and how you would run a cron "every minute or so" with only asterixes.