I have used Perl and a free standalone NT program named blat.exe to send mail from a windows client using a remote SMTP server. I recommend it.

Understand that no matter what type of mail client you use, your SMTP server will need to have its mail relay feature turned on. Most mail servers turn this feature off for security reasons.

In my experience if mail relay is turned off then the SMTP server will give nondescript error messages that don't really identify the problem. If you have not done so you may wish to verify that your SMTP server has its mail relay feature enabled.