Mailing lists are useful for many purposes, but their usual structure — a collection of people belonging to each list — can sometimes be awkward to manage. One example would be mailing lists for students doing classes at a university; there is a central database which tracks which classes each student is in, but to construct conventional mailing lists you'd need to extract a list of students in each class and maintain a copy of the list for each.
Instead, I'd like a piece of software that behaves like a mailing list
server but constructs its lists at delivery time based on database
queries (where the database might just be a flat text file, or it could
be a remote SQL database, or it could be an LDAP system using attributes
to say which lists each person is in). I could thus send mail to
email@example.com without needing to construct the lists first.