A distribution list is a special type of alias, where the target is a group in your address book. Any message sent to the address is forwarded on to all members of the group. This makes it easier to manage an address such as
firstname.lastname@example.org that needs to be distributed to a group of people.
Distribution lists are designed for internal use, where you know all of the recipients. They are not designed for public mailing lists (there's no way for people to subscribe or unsubscribe) or for sending bulk emails like newsletters (there's no way to detect which addresses bounced and no way for users to unsubscribe).
To create a distribution list:
If you tick the Internal option for a distribution list, only messages sent from FastMail accounts will be allowed. Messages from elsewhere will be silently discarded. This can be useful if all the members of the group are FastMail users, and you are not expecting anyone from outside the group to send messages to it.