All of my emails from my inbox apart from the ones dated from last friday have disappeared from all of my devices (iPhone, iPad, and online).  I managed to recover some of them (from around mid September) by going to my trash folder.  This is not the first time this has happened - it also happened earlier this year.  How do I recover all of my missing emails (there's quite a lot, going back to c2006) and how do I prevent this from happening again?