Most mail arrives in my inbox now 'opened' and spam filter not applied

I installed SpamSieve, and all was going well, and it was working perfectly. Now nearly all mail arrives ‘opened’, and spam filters don’t appear to be working. The only thing I have done in the interim, is open Mail on my other computer (I run two offices) which does not run SpamSieve. I have since closed Mail on that computer, but the computer is still on. Not quite sure what to do. I presume because all email is arriving ‘opened’ it is not being filtered?

Please advise

Yes, Mail does not apply rules (such as SpamSieve) to messages that are already marked as read. You need to figure out which Mac, device, or server is doing that and make it stop. Do you have any Mail rules that mark messages as read?

Thanks - will go through a process of isolation to identify.