Can I also see mail that I send via Yahoo Webmail in Outlook or Apple Mail?

When you use Yahoo webmail, your sent mail will also be placed there in the Sent Messages folder. So you cannot find it in e-mail software such as Outlook, Apple Mail or Thunderbird. If you want to find the messages that you send via webmail, you can always set a copy to be sent to your own mail. Keep in mind that your mailbox will fill up faster.