maildir-bulletin takes a message on standard input, i.e. it runs in the same fashion as a mailing list program, and puts the message in every user's Maildir. The message is created once and links are created from all user's Maildir's to the original. This saves greatly on disk space usage and time. Delivering a 1K message to 100,000 users should only take a few hundred K of disk space and take ten or twenty minutes. If you have an alias to expand to 100,000 users the same 1K message will take 100M of storage and take several hours.
