Hi - This seems to have been happening for quite a while but I just discovered it. It seems that Contact Form 7 won't send to email addresses that have multiple periods in them.
For example, a customer of mine has an address "firstname.lastname@example.org" and he had never received emails through the site. As a 2nd address, I receive them fine.
I just searched the threads and have seen this reported before but the topic was closed. I know Road Runner email accounts all over the US use this format.