1. Check your email server’s settings to see if an auto-delete feature has been enabled.
2. Check your Outlook Rules to make sure that emails are not being moved to the trash folder.
3. Check the spam lists as emails could be automatically deleted if the content is identified as spam.
4. If emails full deleted from your trash folder, check the storage settings and increase the storage if necessary.
5. Review the security settings to make sure that emails are not being deleted due to security concerns.