My previous reply showed a way of moving the sent items folder onto an external hard drive. If storage on your computer is not a problem, a solution would be to create another folder and either move or copy the sent items into that. Right click onto the email address associated with your web...
This may help http://https://www.bleepingcomputer.com/tutorials/change-email-folder-location-in-windows-live-mail/ . You don't say whether or not you onboard storage is low but you could use an external hard drive if it is.