NEW YORK (AP) -- With a simple adjustment in your e-mail software, you can pretend to be anyone. You can send messages marked as coming from BillGates@microsoft.com. The trick, known as spoofing, is a popular method for spammers to hide their tracks -- you'd blame Microsoft Corp. chairman Bill Gates and not the actual perpetrator of junk mail. To close that loophole, Microsoft and Yahoo! Inc. are each developing systems aimed at authenticating senders of e-mail. America Online Inc. is testing a third.