This should not work. respCode should be the first character of the server response. Any number returned that starts with 4 or larger is an error. Any response starting with 3 or smaller is ok.
I had to change the "if(respCode >='4')" to ""if(respCode <'4')".
Here are the server response codes. The respCode variable will contain the first character of the 3 digit response code.
However, I can consistently send mail from arduino via php on my web server using the mail() function.
Then you should install an email server on that computer also. Sendmail and Postfix are pretty easy to set up.