Mailman follows several rules when handling mail to be sent to a Mailman list. When a message is forwarded to the Owners and Moderators it is because one of the rules said to check with them first.
Moderation Messages arrive with five pieces of information included in them. List is the name of the Mailman list that requires moderation. From is the email address that sent the message being held. Subject is the Subject of the email. Reason is the reason why Mailman held the message for moderation. And of course, the message body itself.
Below are the most common reasons messages are held for moderation:
Reason: Post by non-member to a members-only list. The list is set so only members can send to it, and this message was sent by a non-member. This often catches owners, as being owner doesn't give any privileges except editing the configuration of the list. For an owner to send mail to a member only list, they must either subscribe OR add themselves to the 'accept_these_nonmembers' option on the Sender filters page. You can add non-owners to the 'accept_these_nonmembers' to allow them to post without subscribing also.
Reason: Post to moderated list. The sender is moderated. This could mean only this individual user is moderated or that all members of the list are. To change moderation for just one user, find them on the Membership Management page and uncheck the 'mod' checkbox for them. Submit your changes to save the change. To change moderation for all users, scroll down to the bottom and select 'Off' under 'Set everyone's moderation bit, including those members not currently visible' and select 'Set' to save.
Reason: Message body is too big: XXXX bytes with a limit of XX KB. The sender is allowed to send mail, but they size of the mail is larger than the preset limit. The default limit is 40k, which is a fairly large email and allows for small attachments. You can increase the limit by changing the value of max_message_size option on the General Options page. However, email is not the best venue for sending large attachments (a list with 500 members, each receiving a 2MB file means 1GB of email). Rutgers recommends that this value is not changed as too many large attachments going out can seriously degrade Mailman service. Any list causing this kind of degradation may be temporarily shut down to preserve the Mailman service. In most cases, you would be better served hosting the attached file on a web page and point to that web page in an email.
Reason: Message has implicit destination This means Mailman received this message but that the message did not have the listname in either the 'to' or 'cc' fields. This most often occurs because the message is forwarded to the list from another person or list OR because the mail is spam. Spammers often obscure the list of recipients when they send mail. If you had one list subscribed to another, the mail sent to the parent list would be flagged with this reason. To avoid this, you can go to the Recipient filters and add the parent list name to acceptable_aliases option. Submit your changes to save the change.
Reason: Too many recipients This is mostly a spam interference option. Spammers often send mail with large amounts of recipients listed in the 'to' or 'cc' fields. This defaults to a limit of 10 recipients. If you find you are often sending to more than 10 mailing addresses at once, you may want to consider requesting an additional Mailman list.