The reason for this error is usually due to messages are being held until the moderator approves them. It means the message was forwarded or BCC'ed to the list’s address, instead of being sent directly to the list in the TO or CC.
Such messages are generally considered dangerous, so the Mailman default is to require that the original sender make their intention explicit by actually using the list’s real address in the TO or CC address of a mail program.
If you want mailing list to accept implicit destination address, follow the steps below.
1.Login Cpanel account.
2.Under Email, click Mailing Lists.
3. Access Privacy Options >> Recipient filters.
4..Change the Must posts have list named in destination (to, cc) field (or be among the acceptable alias names, specified below) option from Yes to No
5. Then click on "Submit your changes".