[Dovecot] Using dsync to export mail to remote IMAP account

I've read the web/man pages on dsync, but it's not clear to me whether dsync can be used to export (rather than import) a user's mailbox to a remote non-dovecot IMAP account.

(The intention is to use it whenever a user leaves and wants to copy their local mail to Google or whatever. I currently use uw-imapd's mailutil -- it works, but it's dreadfully slow.)

As a first crack at it, I tried (on the dovecot server)

dsync	-o imapc_user=remoteuser
	-o imapc_password=remotepassword
	-o imapc_host=remote-imap-server
	-o imapc_port=993
	-o imapc_ssl=imaps
	-o imapc_features=rfc822.size
	-o imapc_list_prefix=remoteprefix
	-o mailbox_list_index=no
	backup -f -u localuser imapc:

I got this error:

dsync(localuser): Error: user localuser: Initialization failed:
	Namespace '': Unknown mail storage driver imapc
dsync(localuser): Fatal: User init failed

Is exporting mail to a remote (non-dovecot) IMAP account even doable with dsync, or am I completely off-track?

Joseph Tam <jtam.home@gmail.com>