Flag emails in Mail on Mac
An easy way to find email messages is to mark them with a flag. When you do, the Flagged mailbox (a type of Smart Mailbox) appears in the Mail sidebar and the Favorites bar and displays messages you flagged.
Flag emails
In the Mail app
on your Mac, select one or more messages.Click the Flag button
in the Mail toolbar (or use the Touch Bar), then choose a flag. The flag appears in the header of each message.
Remove flags from emails
In the Mail app
on your Mac, select one or more flagged messages.Click the Flag button
, then choose Clear Flag.
Rename flags
In the Mail app
on your Mac, click the arrow 
next to the Flagged mailbox in the Mail sidebar.You must be using more than one flag for flags to show in the mailbox.
Click the flag name, click it again, then type a new name.
For example, you could rename Red to Urgent.
Show flagged emails
In the Mail app
on your Mac, click the arrow next to Flagged in the Mail sidebar or the Favorites bar, then choose a flag.Only messages marked with that flag are shown in the message list.
To show all messages in the Inbox again, click Inbox in the Mail sidebar or the Favorites bar.
You can search for flagged messages. Type a flag name in the search field, then select the suggestion in the Flags category. Or type “flag,” then choose the suggestion in the Status category.
If you use iCloud Drive, your flag names are available on your other Mac computers that have iCloud Drive turned on and Mail selected in iCloud Drive options. See Use iCloud Drive to store documents.