Create a user in your Expedia admin platform using Hostify system email as the user email.
The new Expedia user must be assigned with role Guest experience user/Reservation manager under "Role selection"
(To open the multi-property notifications settings you should click on the user settings (top right corner) -> Manage notifications)
Here is an Expedia article of how to create users https://apps.expediapartnercentral.com/lodging/help/help-article/property/property-administration/create-user-accounts-for-your-property?domain=HOTEL
After the user is created, new messages from guests should start to come in Hostify inbox.