I sent a message to the list, but the server sent a message back saying: "Your message to the mailing list has been transmitted to moderators". Why ??

This simply means that your message, before being sent to list subscribers, was transmitted to list moderators to be approved or refused.

Two scenarios:

  The list is permanently moderated, so that whatever the messages and the archives contain always complies with this list’s aims and netiquette. The main aim of moderating in this case is to eliminate messages which have nothing to do with the subject or have nothing appropriate to offer the community.

  You are used to the list not being moderated, but you receive a message from the server telling you the contrary for the first time. Most often, in this case, the list manager(s) have decided to moderate for several days following exchanges which really had no place on the list but which should rather have been private ( "forceful" exchanges, unsolicited advertising, inappropriate language, etc...)

For more clarity on this subject, consult your list netiquette or contact the persons in charge of the mailing list.