From what you are reporting it seems that there are two problems occurring. The first problem seems to be that there is some output occurring on line 2 of contactus.php. This is likely from some white space that is present on or near this line. Getting rid of the output that is specified should get you past the error about the headers.
As for the definition error this function is defined in the WA_Universal_Email > mail_PHP.php file. So long as the include reference is present for this file and the file is in this folder on the server you should not get this error.
I have opened a ticket for you on this issue so that we can exchange files and some more details. To update the ticket please login to WebAssist.com and visit your support history.