This reminds me a lot of the SUBSCRIBE method [0] in Microsoft’s WebDAV extensions (I think this is used for EAS?). You pass a Call-Back header with a URL that

This reminds me a lot of the SUBSCRIBE method [0] in Microsoft’s WebDAV extensions (I think this is used for EAS?). You pass a Call-Back header with a URL that gets called with NOTIFY [1] [0] https://docs.microsoft.com/en-us/previous-versions/office/developer/exchange-server-2003/aa143117(v%3dexchg.65) https://docs.microsoft.com/en-us/previous-versions/office/de... [1] https://docs.microsoft.com/en-us/previous-versions/office/developer/exchange-server-2003/aa142938(v%3dexchg.65) https://docs.microsoft.com/en-us/previous-versions/office/de...