Here you go, this only looks at making sure that it looks right, not that it necessary is a valid active email account (sorry for the delay, never saw this posted), in this case you post it in the custom javascript screen. In this case I am using question q7, you would need to update the ssi get value to whatever your open-ended question is.
function emailvalidate(parText, parPattern)
{
if(parPattern.test(parText) == false)
{
return "You have entered an invalid email address.";
}
else {return "";}
}
{
address = SSI_GetValue("q7");
pattern = /^([A-Za-z])+([A-Za-z0-9_\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;
strErrorMessage = emailvalidate(address,pattern);
}