The mail that is used in UE4 is universal for linux and windows systems. Looking at the code that is on the page it seems that the trigger might be the problem. You are using the same page post trigger. What happens if you update this to be 'any form post' or 'submit button pressed'? Please give this a try to see if you get a different result. I have seen before where the http_referer is not set and it causes the trigger to fail when it should be passing.
Please post back with the results that you get.