Hi,
if you enable the SMTP ONLY the emails sent by Newsletter will use that SMTP, otherwise Newsletter uses the mailing system of WordPress. But check on main setting to have deselected the “send email directly”, otherwise Newsletter uses an independent method always relying on the blog server.
Of course, if you have an SMTP plugin, setting Newsletter (as is out of the box) to send using WP, emails are sent by your SMTP plugin.
The only limitation is that Newsletter cannot send the textual part and will send newsletter only as HTML.
Stefano.