There are numerous guidelines and best practices around Outlook configuration and using its cached and online mode depending on the client’s connectivity to the Exchange server.
Normally the guidance is to use Outlook cached mode when connectivity to the Exchange server isn’t within the same local network, to ensure a great end-user experience.
However, it’s the opposite if you have a virtual desktop environment and use Office 365. With a virtual desktop, it’s best to keep the data small and use Outlook in its online mode, with no local cache.
The good news: Outlook cached mode is now supported in a remote desktop environment. However, the online mode is still the preferred mode.
Microsoft has released a document “Cached Exchange Mode in a Remote Desktop Session Host environment: planning considerations,” which talks through the options and how to make the right decision for your organization.