Hi my name is ***** ***** I can assist you. The Sent folder in any client is only relative to client that sends it. If you sent it from your Kindle, it won't show up on the webmail page. You might be able to get this to work if the e-mail profile is set to IMAP rather than POP, since IMAP is a direct connection to your e-mail server.
Once your e-mail has gone through the SMTP servers at AT&T, they are not able to be recalled. The speed in which they go through the server make it impossible to do so. If you had a local mail server where you were sending to another person on the same server, such as a corporate environment, it would be possible to recall the e-mail since the e-mail still resides on that server.