You can delete multiple email messages in your Outlook 2013 folders using the Delete command. Hold your Ctrl key while selecting messages to add more than one message to your selection. Delete your selected messages using the Delete command in the right-click menu, the Delete key, or with the Delete icon in the Home ribbon and your message list. You can also automatically delete redundant messages from Outlook Conversations using the Clean Up feature. Removing items from your Deleted Items folder will permanently remove them from Outlook.
Deleting Multiple Messages
Open the mail folder containing the messages you want to delate, such as "Inbox." Hold "Ctrl" while clicking on messages to select each message you want to delete. You can delete your selected messages using a few different methods. Click the "X" icon that appears in the right side of a selected message, click the "Delete" button in the Delete group of the home ribbon, press the "Delete" key, or right-click your selection and then click "Delete." Deleted messages are stored in your Deleted Items folder.
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Deleting Messages with Conversation Clean Up
A Conversation contains every message and response connected to the original email message. Since most responses include a complete transcript history, you can delete redundant messages using Clean Up commands. Click the "Clean Up" drop-down button in the Delete group and then select one of the options. Click "Clean Up Conversation" to delete redundant messages from your currently selected Conversation, click "Clean Up Folder" to delete redundant messages from all Conversations in your current folder, or click "Clean up Folder & Subfolders" to delete Conversations from your current folder and its subfolders. Click "Clean Up" to start the process.