Wait. As I get this, although you're on a linux box that has sendmail you want to use an SMTP server on another machine?
Fill out the SMTP part of php.ini and leave p[ath to sendmail blank if you really do want to go that way.
And be sure that the SMTP server will allow relay from the linux server.
To echo Rick's reply to your question:
Will UBBthreads handle the SMTP transaction with a mail server directly or is it uniquely set up to integrate with sendmail on Linux and something else on Windows?
It's not UBB.threads' job to handle the mail transaction. PHP does that according to your settings in php.ini.
If you want to use sendmail fill that part out, if you want to relay through an SMTP server then fill that part out instead.