According to the Federal Statistical Office of Germany, around 4.9 million people were employed in the civil service in 2019. Of these, around 1.7 million were civil servants and judges, 170,000 were soldiers and three million were public employees.

Also don't forget the yearly cost of windows itself. (And keep in mind that even German tanks run on Windows.)

Edit:

According to https://www.zdf.de/nachrichten/politik/deutschland/it-open-source-bundesregierung-kleine-anfrage-100.html

The German government pays 6 billion per year to Microsoft and Oracle.

Thats 70€ per year, per citizen, or 1200€ per year per civil service worker.

Thats a lot of wasted money when you consider that in a tightly specced environment Linux runs fine just fine for free and the money would be spend on local support companies like SUSE instead of overseas.

While it might not be as much, it still will be something.

I work in a purely windows environment because our main software does not really exist outside of it. The hours of IT troubleshooting for the most inane things I see happening is a pretty penny as well. The newest curiosity is Teams killing my RDP session once it loads in the GUI and the IT team is utterly clueless why. It doesn’t make sense, it doesn’t happen to anyone else and the only way to stop it is to kill the process via taskmanager.

And while a government might not be able to go FOSS, there are tools for communication that aren’t built like Teams.

My SO is in a government job and most of their software is some adaption on SAP or similar. They don’t have any chat apps. They use mails or telephone. They do have Skype, but that thing is a performance nightmare in their environment so they only use it if they absolutely have to.

Same goes for stuff like OneDrive. Even if you could wrangle it enough that it fits data security laws, it isn’t something they use in their daily work.

Postfix with MTA that filters maybe.

This provides very little of exchange's functionality. The closest thing I've seen in the open source universe is zarafa, which crowbars activesync emulation into an imap/caldav/carddav infrastructure, badly I might add, and with 3-4x the complexity, maintenance cost, and attack surface. I wouldn't even recommend it for a small business let alone a government agency with all the compliance regulations they have to deal with.

This is one case where Microsoft owns the market because they legitimately have the best tool for the job.

OneDrive -> NextCloud/OwnCloud.