My brother (who is in Valencia, Spain) CAN NOT send email (REPLY); though, he can RECEIVE (and READ) his incoming emails (Yahoo); as well as surf the 'net. He is using a MacBook 13". Initially, we thought his email account might have been hacked; so, he has changed it. On a non-Mac (Windows) computer laptop/desktops, though, his account works just fine. What gives?!?