I’ve just installed this plugin, I have wp-smtp-mail plugin installed as well. The test thing on the plugin works, however I receive an error when trying to utilize the actual contact form.

The error it self is:

The e-mail could not be sent.
Possible reason: your host may have disabled the mail() function.

http://wordpress.org/extend/plugins/si-contact-form/

solution:

Some web hosts restrict sending mail unless the “From” address is a
valid address on the same domain.
Try step 3 of the “email not working” in the FAQ

http://wordpress.org/extend/plugins/si-contact-form/faq/