REST API for on premise Exchange Server 2019 CU 11

[Rox_Star]

We have offline environment not connected to internet having on premise exchange server 2019 cu 11. I need to show user's inbox unread mail count on a third custom developed application (logged in using same AD account).

Is there any API (REST or SOAP) to access the users mailbox?

Regards

[#2]

REST API was in preview for a while, and recently Microsoft decided to deprecate it: https://techcommunity.microsoft.com/t5/exchange-team-blog/the-end-of-the-rest-api-for-on-premises-ma...
You can still use it for an year or so though.