how to delete thousands of sent messages in my iCloud mail account

Hello tweetybird71,

We'd be happy to help. If you're looking to select all on the iCloud website or in the Mail app on your Mac, you can use Command+A. That should select all and you can action them how you'd like. If you're using a Windows PC, you can use Ctrl+A. You can learn more keyboard shortcuts here: Mac keyboard shortcuts - Apple Support

Thanks.