I'm not sure why the variable reference is not working for you. To help determine the cause it would be a good idea to print out the value within the helper file to see what is being set for the server during run time.
In your WA_SecurityAssist folder in the Mail_PHP.php file go to line 52 and add in a new line just after this line. In this new line add in this code, it will stop the page from processing and let you know what the mail server is set to at this point.
die("The server name is: " . $serverName);
Please test this out with both the entered value and the variable value to see what you get. Post back with your results.