iPhone 6s storage usage I want to ask you why in the usage section of iPhone 6s on iOS 9.2 it says I have 2.6 GB in Mail. However, I have deleted all inbox, sent, trash, etc. There is nothing in Mail. 

How do I get the storage in the usage section to empty this Mail?
This issue might be graphical 'bug'. If you are sure that mail is empty, you can always try to reboot/hard reset device and if the issue is still there, then take a iTunes backup and restore device. https://support.apple.com/en-us/HT204184 should be good article for this troubleshooting step.