A list will contain zero or more 'members' (usually email addresses) which can be messaged all at once. To send a message to all the members of a list, you will usually send an email to the listname at a valid domain.
For instance, if your 'Local Domains' setting is set to mycompany.com;mycompany.org, and your list is called Customers, then you can send a message to all your customers by sending an email to firstname.lastname@example.org or email@example.com
If you want the list to be accessible using other email addresses, then you can use mappings to create aliases for the list.
If you want a list to be accessed from outside your network, you have two options:
•You can tick the Allow incoming mail to be sent to the list option in the list settings. This makes the normal list email address(es) work from outside your network as well as internally. Note that if the incoming mail arrives via a POP3 mail collector, then VPOP3 uses the Accepted Domains setting for the collector, instead of the Local Domains setting, when calculating the default email addresses for the list. This option is not available for Group lists.
•You can create one or more Mappings for the list. This will make the list be available using the Mapping email address(es) to both internal and external users (as determined by the rules for the Mapping). This option can be used to make Groups publicly accessible.