Postbox 2.5 has a new interface for vertical view. By default, unread messages show a blue dot to the left of the message, but do not appear in a bold typeface. If you'd like messages to display in bold when using Vertical View, you can do so by changing a hidden preference in the Config Editor.

To do this, launch the Config Editor, and search for:

pb.thread_pane.vertical.bold_unread

and change the value to true. To revert to the default setting, change the value back to false.