If you have Authentication enabled and the correct user/password and still receive that error it may be an issue with your ISP blocking Port 25 and using Port 1025 or 587 would be alternatives. http://Service.FutureQuest.net/kb165
Thanks. Everyone who sends from this computer is having the same trouble, including my wife, who uses a Verizon e-mail account. Does this change your answer at all, or should I follow your second scenario above?
It would depend. If the machine is on a Verizon provided connection and cannot send mail through the Verizon servers then I would suspect an issue with your Windows Mail configuration/installation or possibly a Firewall issue.
Either way I would recommend contacting Verizon and working with them to at least get your Verizon mail to be able to send and then work on your FutureQuest mail separately.
If the machine is connected using another ISP other then Verizon then I would still try the alternate ports for your FutureQuest mail, I can't speak for any alternatives for Verizon.
If it was only your FutureQuest mail then that would indicate a good chance that your Local ISP has just started blocking Port 25 however as it is affecting more than one provider then it would almost have to be something local to you.