Module Webform won't send email to certain address
I was having this same issue. I contacted my host (Hostmonster) and found that there is a setting in your Zone Editor that can fix this issue. 

Under MX (Mail Exchanger) I selected Remote Mail Exchanger and under the section that says "Points to" I put exchange.domain.com (replace domain with your domain name). I did this because we are running all of our email through an exchange server.

Try calling your host and explaining the problem.

I hope this helps!