You may want to use a disk space utility to see which folders are causing the issue, but the following 4 folders are most likely to be using all of your disk space:

\MDaemon\Users

\MDaemon\Backup

\MDaemon\Logs

MDaemon\Public Folders

If it is old log files or old zipped backup files which are consuming your disk space, you can delete them or move them to a different drive or medium. If it is mail or information in the "Users" folder or "Public Folders" folder, you will need to find out what users are taking up the space and ask them to remove the mail.

You can move MDaemon's mailboxes to a different drive by following this knowledge base article: Moving MDeamon's Mailboxes.

You can move MDaemon's logs to a different drive by following this knowledge base article: Moving MDeamon's Logs.

Also, you can turn on old mail pruning (Setup | Domain Manager | Options | Account & Mail Pruning and Setup | Server Settings | Pruning) to have MDaemon remove mail that is older than a certain number of days.

To prevent users from accumulating too much e-mail in their mailboxes, you can activate usage quotas on a per-user basis. When editing a user account, click on "Quotas" in the left-hand navigation menu. You can set a maximum number of messages stored at once and/or a maximum amount of disk space each user is allowed to use.