Why doesn't one of my subscribers receive copies of his or her own messages sent to the list?
If the subscriber in question uses Gmail, that's probably the issue. See our topic about Gmail mailing lists for more information.
If the subscriber doesn't use Gmail, the information below will probably help.
Mailman's "nodupes" option
Mailman's "Membership Management" screen shows the subscribers to your list and the various options set for each subscriber. One of these options is the nodupes checkbox.
The "nodupes" option tries to make sure that subscribers only receive one copy of a given message. It can sometimes be useful: it prevents recipients from receiving two copies of a message when someone on the list uses "Reply to All" to reply to a list message, for example. (That's because if firstname.lastname@example.org sends a message to email@example.com, then firstname.lastname@example.org receives that message and clicks "Reply to All", the reply would be sent "To: email@example.com; CC: firstname.lastname@example.org". That would normally cause email@example.com to receive two copies: one directly from the mail program of firstname.lastname@example.org, and one from the list server. The "nodupes" setting prevents the list server copy from being sent.)
However, if a subscriber sends a message to the list and also includes his or her own address in the "To:" or "CC:" fields of the message — that is, if the subscriber also sends himself or herself a direct copy of the message — Mailman will notice that the subscriber has already received a direct copy and will not send another.
This occasionally leads to people worrying that their message didn't get sent to the list. If that's a problem, and the subscriber wants to continue also sending a direct copy to his or her address, you can turn off the "nodupes" setting for that subscriber. The subscriber will then receive two copies of such messages (one directly from his or her own mail program and one from the list).