> Microsoft Exchange Server, there used to be an on-premises version of this but MS stopped supporting it

After many enterprise customers refused to migrate Exchange Server SE exists since 2025 and will be supported until at least 2035.

Though given how the quality of their updates are going one could still say that it's "not supported", but at least they are charging money for it!

There's Skype for Business SE too. They've recently brought back the group chat feature.

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