Mails being marked as read automatically

Yesterday I upgraded to the most recent version on F-Droid, 6.511. Now most (but not all) incoming emails are immediately marked as read. I cannot yet really see the pattern in what is marked and what is not - a test mail sent to myself came in correctly as unread, as did the signup confirmation mail from this forum, but most mails are greyed out right away. Is there some setting I could look into?

Scratch that, this seems to be some issue with my home server that is online 24/7 and which has apparently started marking mails for that account as read on the mail server once it retrieves them.