Strange e-mail issue

We are running Exchange 5.5 SP4, and all seems to be normal with our Internet mail, except we have approximately ten people who cannot e-mail to us (we can e-mail them). The NDR they get says that the maximum time for delivering the message has expired.We successfully receive e-mails from hundreds of other domains with no problem. I do not know what else to check at this point, nor do I have any sense of which direction to pursue.