Outlook 365 mail unwanted archiving

Hi everyone. I'm new to Outlook, and new to this forum, so please be gentle! My wife and I have email addresses associated with our charity work, and these have recently been re-assigned to an Office 365 environment, using Outlook.

A number of settings issues have arisen and been successfully dealt with but one issue is still stubbornly refusing to be solved. Some Emails that have been read seem to be archived involuntarily,  with the result that a complete chronological list of incoming emails no longer exists. and I have to visit the Archive folder to view them.

Is there a way to disable automatic archiving, and why anyway is it only happening to some?

Thanks in advance.