Mail notification sound but no mail..

So I’ve experienced this a few times now and never knew what app it was coming from but today I checked the notification sounds on my apps and it turned out to be the mail sound, after checking my mail I saw that there was one but from yesterday so why did I hear the notification sound today?! Is this a bug or something? I turned on banners for mail for now so I hope it doesn’t happen again... sorry for my English!