Flag emails in Mail on Mac
An easy way to keep track of email messages is to mark them with a flag. When you do, the Flagged mailbox (a type of Smart Mailbox) appears in the Favorites bar and the Mail sidebar and displays messages you flagged.
Flag or unflag emails
In the Mail app on your Mac, click the disclosure triangle next to the Flagged mailbox in the Mail sidebar.
You must be using more than one flag to show the flags 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
Only messages marked with that flag are shown in the message list.
To see all messages in the Inbox again, click Inbox in the Favorites bar or the Mail sidebar.
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. See Use iCloud Drive to store documents.