If a list subscriber complains of missing list mail, first search the subscriber list for the address they contact you from. The first 2 possibilities are their account is bouncing, or they have recently changed email addresses, and did not update their subscription address.
If their address appears on the list, and it is not marked as bouncing, either the subscriber or their ISP may have recently implemented or changed spam filters that are causing list messages to get caught. When possible, the subscriber should check any Junk Mail or Spam mailboxes they may have. Additionally, they should add email@example.com to their Address Book, as many ISPs whitelist, or automatically accept, mail from addresses in user's address books.
The subscriber should also contact Technical Support for their domain for more information. Most ISPs will add mailing lists to a system-wide whitelist at a user's request.