Do you mean you want the connection to be made directly to an SMTP server, instead of sending via the mail command on the server?
If so I'm afraid there's not yet an easy way to do this via the admin interface. You can do this yourself with the following steps:
1. Set QA_EXTERNAL_EMAILER to true in qa-config.php.
2. Rename the qa-external-example directory to qa-external.
3. In qa-external/qa-external-emailer.php, modify the qa_send_email() function to match that in qa-include/qa-util-emailer.php.
4. Look at the parameters at the top of qa-include/qa-class.phpmailer.php to see what you need to provide SMTP sending (including $Mailer and a whole group of SMTP parameters).
5. Modify your qa_send_email() function in qa-external/qa-external-emailer.php to set the necessary parameters, based on what you saw in the previous step.
Note that you *could* also just modify the email sending function in qa-include/qa-util-emailer.php, but this means you'd lose your changes if you upgraded to a newer version of Question2Answer. This way your changes will be compatible with future versions.