I wonder if someone can help here!

I have recently discovered that many of my old emails are getting automatically deleted - in fact when I pay close attention, all emails that are older than 10 days old will be automatically moved to the "deleted" folder. I have checked my settings and I haven't opted for automatic email clear-up and I reaaally wonder why this is and it really is creating a big mess for me. Please help! I can't have all my emails being deleted all the time...

Thanks!