The forms action should left blank. when left blank the form will post to itself.
The code for precessing the form and sending the email should be on the contactus.php page already.
If you leave the forms action blank, are you redirected to the thank you page?
If so, this indicates that the code for sending the email is being triggered. If not we will need to find another avenue to debug.
I am going to open a ticket for you so we can look into this further. I would like for you to reply to that ticket by logging into the support history page at:
Please include a copy of the contactus.php page and let me know whether you are redirected to the thank you page after submitting the form.