I’ve a folder with a number in bold on the left: I assume there is unread messages in this folder.
If I do right clic on this folder and “mark all messages as read”, tthe number remains present in bold: nothing has been done.
It has worked on other folders but not on some others.
Any clue?
I think that the number in bold is on the right. 
But this may be an effect of Conversation mode. If you have a conversation that has unread messages in the Inbox, the same conversation is also displayed in the Sent folder, so it will indicate unread messages there as well. However, right-click and Mark as Read on the Sent folder will not change the unread marker as the unread message is actually in the Inbox. Go to Menu > View > Conversations and by disabling conversations see if the behavior changes.
If this is not the issue, close eM Client and run C:\Program Files (x86)\eM Client\ DbRepair.exe. It may be that there is some corruption.
I’ll see this and keep you informed.
I tried to “mar all read” on sent box: a bold number remains (on right of the text in the left windows… )
I tried also “mark all read” in a subfolder but no change.
disabling the “conversation mode”, I’m back with just a few messages as “not read” but I loose a functionnality.
=> in conversation mode, is there a way to localize a message from conversation adn/or to display only unread messages?
When conversations in all views is selected, the unread count is indicating unread conversations, not unread messages. In this mode marking the whole folder as read will only apply to messages that are not part of a conversation, or where all the unread messages in that conversation are in the same folder. If one of the unread messages is in another folder, the conversation will not be marked as read.
But there is an in-between setting: Conversations in message detail only. That includes the conversation members in the message pane, (right window ) but not the message list. So some of the functionality is there, but the message list is displaying single items. You will be able to see which messages in the conversation are unread as this mode displays unread messages, not unread conversations.
One thing i have found with some Unread messages in EMClient that just don’t won’t to seem to change to read is to dblclick them to open them, and look at the (very top) of the message and you will normally see (tiny little dots) moving across the screen.
Leave the message open untill the dots stop moving or dissapear. It can take along time for some emails to change status in EMClient to Read particularly imap mail. There can be lots of hidden remote stuff in email that can just take forever to download which then keeps the email as unread till its all done.
You can also set the (Mark Messages Read) delay in (Menu / Tools / Settings / Mail / Read) section. I set mine to 5 secs. But if there is a big graphical email sometimes it won’t change to read for alot longer than that depending on what content is in the email.
I have recently discovered that when a new message arrives with a link, and you click on that which opens the web browser before the message is marked as read, the message remains as unread.