Your account’s already set to automatically save your sent emails in the Sent folder. So you don’t need to do anything!
But you can change this setting if you want. You can choose to not save your sent emails. And you can switch back again, too. Anytime.
To find this setting, click Options in the upper right corner of your Mail page and select Mail Options.
The Sent folder appears in the folder list along the left side of your Mail page. See it? Click it for a view of all the messages you’ve sent.