If the list is set up for subscription moderation, a moderator has to approve the user's subscription request. The process starts with a user -subscribe request, followed by the normal user confirmation. After this, ezmlm sends a second confirmation request to the moderator(s). If any moderator replies to this request, the user is accepted as a subscriber.
You may send the user a message to request further information. However, this will reveal your address/identity to the user.
Users may unsubscribe at any time without moderator assistance.