Why my email disappeared

Hello,i have a question you make me happy by answer it last week i noticed one of my email is absent in my sent box it was one of my very inportant message i guessed maybe it is moved to trash by mistake so i opened the trash folder and found it there i was so happy. Before doing any thing tried to open it and see the message body the message opened tried to being load but vanished after a few second and my trash became empty it was so frightening for me i tried a lot to retrieve it by using help service searching any where in mailbox,nothing changed now i really wanted to know why this happened i didnt tap any delete button i should add this that i was using my iphone s6 mail app maybe its been deleted by mistake but it wasnt deleted from trash but after opening it became deleted any one knows why?