Simply select any email at once
If you want to delete messages in bulk, mark multiple messages as read, archive the mail folder or send a group of messages to the Junk E-mail folder, select multiple e-mail messages, or view all e-mail messages in a mailbox folder Selecting. Use the Select All option to select a group of messages in Outlook on the web.
The instructions in this article apply to Outlook.com and Outlook Online.
Select all emails at once
To select a message group:
Navigate to the folder containing the desired emails.
Select Choose them all (it’s a light gray circle with a check mark above the message list).
Each email message in the folder is selected and a check mark appears next to each message.
To deselect a message, uncheck the box next to the message you don’t want to be included in the group.
Do whatever you want with the selected emails, such as delete, archive, move to another folder, or mark them as read or unread.
Use a dedicated email client for even more flexible sorting and selection. For example, if you use Microsoft Outlook, you can back up your email for safekeeping.