The pictures download from the internet when you view the e-mail online. Once downloaded they should be there permanently. There will probably be a more detailed reply but in essence this is the reason. (I think)
Hands-on: Samsung Galaxy S8 review
1995-2015: How technology has changed the world in 20 years
Samsung's beautifully designed Galaxy S8 makes for better VR experiences too
47 iPhone camera tips to help you take better photos
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.