My reading of the maildir specification (such that it is) says that the
filename before the colon should be unique. Although ... despite what
people say, it seems like Maildir is not really designed to be a mail
_store_ (someplace where you keep your mail long-term) but more of a
mail drop box (someplace you put messages so someone else can come along
and grab them). Like, the original Maildir specification did not permit
subfolders (it looks like there is something called Maildir++, which does).
[family.redbarn:amd64] ls -lR Maildir/.w.spam.sunbelt
total 21
drwxrwxr-x 2 vixie staff 6 Jan 24 04:06 cur/
-rw-rw-r-- 1 vixie staff 10 Jan 24 04:06 dovecot-keywords
-rw-rw-r-- 1 vixie staff 291 Oct 8 04:43 dovecot-uidlist
-rw-rw-r-- 1 vixie staff 4636 Feb 29 03:44 dovecot.index.cache
-rw-rw-r-- 1 vixie staff 1184 Jan 24 20:19 dovecot.index.log
drwxrwxr-x 2 vixie staff 2 May 10 2015 new/
drwxrwxr-x 2 vixie staff 2 Mar 4 10:43 tmp/
Maildir/.w.spam.sunbelt/cur:
total 275
-rw-r--r-- 1 vixie staff 4155 May 10 2015
1431239881.M607591P59215.family.redbarn.org:2,a
-rw-r--r-- 1 vixie staff 6113 May 10 2015
1431239881.M611137P59216.family.redbarn.org:2,a
-rw-r--r-- 1 vixie staff 1612 May 10 2015
1431239881.M614599P59217.family.redbarn.org:2,a
-rw-r--r-- 1 vixie staff 237451 May 10 2015
1431239881.M617919P59218.family.redbarn.org:2,a
Maildir/.w.spam.sunbelt/new:
total 0
Maildir/.w.spam.sunbelt/tmp:
total 0