Maildir, as opposed to emails stored in one big file, does cause some disk space to get wasted, but in past 10 years, it is immaterial. Even million empty email

Maildir, as opposed to emails stored in one big file, does cause some disk space to get wasted, but in past 10 years, it is immaterial. Even million empty emails, each taking 4kB of disk space, would take only 4GB in total. The small files aren't a concern for saving disk space: the big ones are. Maildir has the advantage that searching, adding and deleting emails is very fast, much faster than if those operations were made on single big file. Worth it.