You can mark emails as pinned using the pin icon in the top right of each email summary in the inbox list view. This icon can be made visibile with these settings. View

You can show pinned emails at the top of the inbox view using 'Filters'. Open the action menu (circle with 3 dots button) and then tap on 'Filter'. Choose the 'Pinned' filter.

You can also create your own 'Split Panels' to group emails in different categories, such as pinned and unread etc. More