A mailing list is a list of addresses of people who want to receive mail on a particular subject, or from a particular sender. At one time the term applied exclusively to lists of addresses to which postal mail would be sent, and it is still sometimes used in that sense. Now, however, it most often means an electronic mailing list or email list, which is a method of sending the same email message to a group of recipients at once.
Electronic mailing lists can be implemented in various ways. The simplest form is merely a list of the email addresses of the list members, which can be copied or pasted into the address section of an email message. However, this "manual" form of electronic mailing list is fragile and can be a problem to maintain. Therefore, specialized software for running electronic mailing lists was devised. Such software typically maintains a database of addressees, and has an interface to allow authorized people to add or remove addresses from the list. It will also typically have a separate list of those authorized to send email to the addressees. Often a list title is automatically added to the subject line, and pre-chosen text may be automatically added to the start or end of each message. In some cases some or all messages may be forwarded to a moderator for approval or rejection.
Most mailing lists are primarily or exclusively text-based, but some allow non-text files such as images or programs to be distributed.
A mailing list is distinct from a newsgroup in that the messages all go through a central site, and are sent directly to those on the list of addressees, not to anyone who chooses to access a group on a server.