decade 2 Junior Poster in Training

I created a program using php that will attach a file if you click "attach" button. the problem is that i always receive this message when i try to attach a file even though I didn't create an error message that states this "attachment failed. this may be due to a proxy or firewall" message.
i've already check the firewall setting but this message keeps on appearing every time i test my program. can someone explain or teach how to deal this problem.