The grommunio project functions well as a drop-in replacement for Exchange. The developers connect components to the platform the same way Microsoft does, and they support RPC (Remote Procedure Call) with the HTTP protocol. According to the developers, grommunio also includes numerous interfaces of common groupware such as IMAP, POP3, SMTP, EAS (Exchange ActiveSync), EWS (Exchange Web Services), CalDAV, and CardDAV. With such broad support, grommunio integrates smoothly into existing infrastructures.
There are a variety of usage models, from paid support with cloud hosting through to community free options. Remember though that payments is what keeps the project innovating and improving. Self-hosting will also help achieve full data sovereignty, and with Linux being used as a hosting platform this also provides a lot of flexibility and scalability.
See An open source alternative to Microsoft Exchange
Microsoft Exchange has for many years been nearly unavoidable as a platform for groupware environments. Late in 2020, however, an Austrian open source software developer introduced grommunio, a groupware server and client with a look and feel familiar to Exchange and Outlook users.