[Dovecot] backup using rsync

i'm trying to backup the dovecot Maildir files over a cifs share.
source is a debian build, share is a readynas. i've been getting
rsync: mkstemp 'filename goes here' failed: Not a directory (20)
errors. rsync is v3.0. i believe this is related to the dovecot
file naming convention that includes characters such as ':' in the
filename. anyone else seeing similar behavior? a workaround? i
can go to different backup strategies, but rsync is cheap and
effective for my needs. thanks,

-SM-