Outlook "Memory" Issues When Expanding Folders or Sent Mail Stuck

The fix for this type of issue is to disable the Use Cached Exchange Mode - specifically the Download Shared Folder option.

To disable, navigate through File -> Account Settings - Account Settings -> click the account -> More Settings -> Advanced and uncheck Use Cached Exchange Mode:

The trade-off for disabling this feature is that cached Exchange mode is disabled, so e-mail will no longer be downloaded to the system. If cached-mode is a requirement, then the user must clean up their folders and sub-folders by deleting what is unnecessary. 

More information regarding this issue can be found here: Insufficient memory errors and sent mail stuck - Outlook