Make sure you have send authentication enabled.
Go to Tools, Accounts, select that account, Properties, Servers.
The box for "My server requires authentication" must be checked.
--
Gary VanderMolen, MS-MVP (Mail)
"mhappies" <(E-Mail Removed)> wrote in message news

786EF2A-82A9-4418-8B17-(E-Mail Removed)...
> So I followed all the instructions on how to set up my aim.com account. I can
> get my inbox but when I try to send something I get this:
>
> The message could not be sent. The authentication setting might not be
> correct for your outgoing e-mail [SMTP] server. For help solving this
> problem, go to Help, search for "Troubleshoot Windows Mail", and read the
> "I'm having problems sending e-mail" section. If you need help determining
> the proper server settings, please contact your e-mail service provider.
>
> Account: 'AIM', Server: 'smtp.aim.com', Protocol: SMTP, Server Response:
> '556 CLIENT AUTHENTICATION REQUIRED. USE ESMTP EHLO AND AUTH.', Port: 587,
> Secure(SSL): No, Server Error: 556, Error Number: 0x800CCC78
>
> THe instructions from AIM said to put in port:587 which I did but I'm still
> unsuccessful. Any help?