Thank you for your response.
This is what a simple webmail (and the advanced Outlook) does:
When I choose a folder (“Inbox” in this example), it shows the mail list, read & unread mails remain untouched, the reading pane is blank.
When I click a mail to read, its content is shown in the reading pane, and the mail is marked as read.
This is what eM Client does:
When I choose a folder (inbox, junk, trash…), it not only shows the mail list but also opens and marks the first mail without my consent.
My setup:
Again, some similar complain in the past:
Do not automatically show (and mark as read) first mail - Feature request - eM Client
First email of folder automatically opening - Frequently Asked Questions - eM Client