So, I have several emails in my inbox on email client (Postbox or Thunderbird) and can see the ciontent of one in preview pane.
Without opening any of them, next time I look with the intention of reading them, they've been deleted and cannot be recovered.
None are spam.
Are your mail clients seto to receive mail via POP3 or IMAP?
On POP3, the default behaviour is to download the email from the server and then delete it from the server. If one client is doing that, then the email will be unavailable on any of the others.
Set to Imap