I used to have an Exchange mailbox that had a small “on-server” mailbox size limit. I worked around the size limit by “disabling roaming” which changes the default delivery location for RSS items. With this setting, Outlook created a new PST file Named “RSS Feeds” and stored the posts there. To change the default delivery location of RSS Feeds, open the registry editor and add the following or create a text file ending in “.reg” and the double-click to import:
Windows Registry Editor Version 5.00 [HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Outlook\Options\RSS] "DisableRoaming"=dword:00000001