Maildir is a huge improvement over mbox for storing local mails of users. Why? I will not go into a long explanation about this, because it is not the scope of this post, but just consider that saving each mail in its own file on the disk (Maildir) opposed to saving all mails in a single file (mbox) is much faster. Not only faster, but it is also much easier to manipulate the files (that are individual mails) on the system. For more details on maildir vs. mbox you can see http://www.courier-mta.org/mbox-vs-maildir/
Now, even if Maildir is much faster for many emails kept on the server, when we reach a huge number of files in a single folder the access times to that folder will be considerably slower. Now I am not talking here about a couple of hundreds of mails, but some huge mailboxes with thousands of mails and huge sizes (over 3-5GB in size). You will be amazed that there are peoples that will do that… They are most probably using IMAP and keeping all their mails on the server, or even POP3 and saving one copy of each mail on the server. （更多…）