A mailing list is a list of email addresses that receive one and the same message simultaneously. When you send an email message to the mailing list address, your message will be re-sent to all the email addresses on that mailing list automatically, but none of the receivers will know who the other recipients are. In the general case, one has to join a list, but occasionally email addresses are added manually without the approval of their owners. Depending on the specific mailing list management software, you may also be able to greenlight new mailing list subscribers, so users will not be able to sign up for your mailing list unless you authorize their request. The mailing list functionality is pretty valuable if you want to send newsletters on a regular basis or some other kind of periodic notifications to clients, as you will need to send a single email and all the mailing list subscribers will receive it immediately. This way, you will not have to write lots of mailboxes manually.