Are they both POP accounts? What is your setting in Tools>preferences>general?

Are they both POP accounts? What is your setting in Tools>preferences>general?

Yes, they are both POP accounts.

Setting is set for Mark as read immediately on display.

But I am not even displaying it.

I can download 50 messages. I seem them all being listed as downloaded in the window pane. All in bold face because they are unread.

As soon as all 50 are finished downloading, instantly they are marked as read. I don't have to do anything.

This does not happen on the other account.

Do you access the account on another device, such as a phone? Do you have Bitdefender, and if so, is the BD anti-spam option enabled for this account but not the other?

Do you access the account on another device, such as a phone? Do you have Bitdefender, and if so, is the BD anti-spam option enabled for this account but not the other?

I do not.

I have come across someone else with a similar problem, actually a couple of people. In their case, they had previously set up one of their mail accounts in another email client. One had used 'Mail' and another has used 'Outlook'. That other email client was still working in the background - they did not know it was still working and accessing server. But, it was found to be the culprit. They removed the mail account from that email client and closed it.

Have you ever set up that particular email address using another email client?

Have you ever set up that particular email address using another email client?

I have not. I have only ever used Thunderbird.