Hmm, ok .. .you might want to double/triple check all the permissions on your mailboxes.
you probably have set this:
postconf -e 'virtual_uid_maps = static:5000'
postconf -e 'virtual_gid_maps = static:5000'
Make sure the maildir /var/vmail (for example) and everything in it is owned by uid/gid 5000 and dovecot also uses that 5000 (vmail) user (check master.cf)