Problem: You have a contact us form hosted on your website sending an email to your local domain, but the email never leaves your web server because your mail server is hosted elsewhere. The mail works fine for all other email domains (your customers receive confirmation emails), but your own domain fails to arrive (cc'ing to yourself).
This is really mind boggling problem, and it took me a while to find a solution that works in my environment. In a nutshell, there are a number of solutions but in certain environment first few options doesn't work.