I have both Apple Mail and Thunderbird. I can send mail on Thunderbird but I can't with Apple Mail. I'm using Cox internet and actually many people under this ISP seem to share the same problem. It's because Cox close Port 25. But this doesn't make sense because I've set the SMTP the same on both mail clients: smtp.west.cox.net and port 25 and Thunderbird works flawlessly. Still I tried switching to port 995 on Apple Mail, but I still have the problem.