As more and more Internet Service Providers (ISPs) have to deal with the increasing amount of spam e-mail, many are beginning to block certain e-mail addresses. The problem is that rarely do they send a bounceback e-mail saying the the e-mail is undeliverable, so you think that the e-mail went through. Guess again!
If you suspect that this is happening to you, call you web hosting service and work with them to determine if the e-mails are indeed leaving the server. If the web host verifies that they are leaving the server, then this is an ISP problem and you will need to work with that ISP to get the e-mail addresses unblocked.