Yes, but this primarily concerns configuration information of your SYNCING.NET user account and your SYNCING.NET "Sync Folders" and "Outlook Groups". Your Outlook data and your files are always stored locally on your computers. The data transfer between your computers is naturally cryptographically secured. Nobody can see the content of your files and data.
If you are using the Store-and-Forward feature (accessible under SYNCING.NET “Configuration -> Options -> General Settings”), the cryptographically secured change requests from the last(!) computer online in your SYNCING.NET "Sync Folders" or "Outlook Groups" will be additionally copied onto our server and stored until another computer comes online alone and downloads the change requests from its so-called Store-and-Forward Inbox. In other words, this data is being stored locally on your computer and on our server.