iCloud 
Flag messages
In iCloud Mail, you can flag messages for special attention in your message list. For example, you might want to flag all RSVPs to a specific email invitation so that you can tally responses later. You can then sort your message list to quickly access the flagged messages.
Flagged messages are displayed with a flag icon 
in the message list.
Flag messages
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In the message list, select the message or messages you want to flag.
You can Shift-click to select multiple adjacent messages or Command-click to select multiple nonadjacent messages.
Click the Flag button in the iCloud Mail toolbar.