Sounds like they're getting filtered or blocked or something. You don't mention what email address you're talking about (whether it's an ISP account, an account with a domain you host with us, a freemail account, or what), but send in a support message about it if you haven't already.
A few suggestions:
1) Check with your ISP to see if they're doing any sort of filtering or blocking that might be affecting these messages.
2) If you have server-side or client-side filtering setup, make sure we're not getting tagged as junk mail somehow.
3) Add a secondary contact that's a different email account with another provider, and see if that one works.
4) Have support check the mail logs and see if the messages are being accepted.
Lastly, is the address an email address at one of your domains we are setup to host, but which is actually hosted elsewhere?