This function is only available for the Advanced plan.
You can mark the significant email or email to view later as important. Also, you can collect and only view the emails marked as important.
Mark email as important
Emails marked as important have 
on the email list.
Mobile
Open the email to mark as important and then tap 
to mark.
PC Web
Open the email to mark as important and then click 
to mark.
To remove the mark that indicates the email as important, click once more.
Collect and view emails marked as important
You can filter to only view the emails you marked as important in one place.
Mobile
- Tap
in Mail. - Tap
to filter and only view the emails marked as important.
PC Web
- Click
on the top. - Click 'Important' on the top left in Mail.