My main reason to move from Mail-in-a-Box[1] to AWS WorkMail[2] to finally Microsoft Office 365[3] was that there is no other implementation which supports all MS Outlook features like native MS Exchange.

Are there any (Self-Hosted?) alternatives nowadays?

1: https://mailinabox.email 2: https://aws.amazon.com/workmail/ 3: https://www.microsoft.com/en-us/microsoft-365/exchange/excha...