The only way we've found to do this is to create a temporary DA server using mbox, restoring the domain(s) to that, and then converting the temporary server to Dovecot, and then backing that up and restoring it to your live server.
You'll need port 587 opened in your firewall, and DA's exim install will automatically use it for authenticated senders. The configuration is already there.
Just make sure you've got the latest (version 2.0) version of the SpamBlocker exim.conf file. Look at the first few lines of exim.conf, or grep the file for 587.