How to Select All Email Messages at Once
To select a group of messages:
Go to the folder that contains the emails you want to select. Choose Select All (the light grey circle with a checkmark in it above the message list). Every email message in the folder is selected, and a checkmark appears next to each message. To deselect a message, clear the checkmark next to the message that you don’t want to include in the group. Do what you want to the selected emails—delete, archive, move to a different folder, or mark as read or unread—just as you would for a single email. Your action will apply to all checkmarked emails.
