There are two settings to review: 1. at tools>account settings>synchronization&storage, you can set to never delete emails, or to specify a retention period when they will be automatically deleted. 2. at tools>account settings>server settings, you can specify whether to delete immediately or move to trash, and also whether you want to empty trash on exit. To only delete after a final read, you should set the synchronization option to 'never delete' and untick the empty on exit option.