Disappearing e-mail

I use Outlook on my iPhone, iPad, and iMac to read and send email using a Gmail and a university email address. For some reason, the iMac and iPhone at some point in the recent past began deleting a lot of my email as soon as (or shortly after) I read it. The iPad seems to retain most of the email. My preference is to leave the mail on the servers rather than delete it. I definitely do not want it deleted on my devices. I am relatively experienced with changing settings on my devices, as long as I know which settings to change. I've searched for solutions to this problem on the web but haven't found useful answers. I need assistance resolving this problem, if anyone can give it. Thank you, Hugh Ruppersburg