Subscriber says he is getting messages, but his posts are rejected.
This can happen when the subscriber is receiving email at one address, and posting from another address. The From: address must match the address on the list exactly.
For instance, if Joe signs up as firstname.lastname@example.org, and later his email address changes to email@example.com, then his posts will be rejected. In this example, maybe Joe's employer made the change without Joe realizing the difference. His old email address is forwarded to his new address, while all new emails from him are sent from the new address.
For help resolving this issue, please see this post.