First is to login to the OX App Suite via: https://us.appsuite.cloud/appsuite/

To search for an email, use the search function in the upper left corner.

As soon as you start entering a search word OX App Suite offers you suggested searches via auto completion. 

You can choose to search via subject, in mail text, in mail attachment file names or via email address by clicking on the appropriate search.

You can also select which folder to search in by clicking “Inbox” the right of the search box and click “All folders” to search in all folders or select a specific folder to search in.