Maildir vs. Mbox
Maildir is useful in that it doesn’t require locking so the SMTP, POP3 and IMAP servers can fiddle around without too much worrying, but when someone has 4 GB worth of it in a single folder that all by itself is already over 10 MB in size, that advantage becomes more something of a burden and makes you wish that it was all one big mbox file, instead. Especially if you want to keep that data in sync with a copy elsewhere using rsync. It will do it, sure, but if does not appreciate having to go through huge folders like the one mentioned above and memory usage goes a little crazy.