Our host is migrating our server instance. They usually (99.9999% lol) go off without a hitch. 

As we don't have dovecot running elsewhere, I'm assuming doveadm is the wrong tool.

If we want to make a one-off backup prior to the migration, is shutting down postfix and running 

tar czf mailstorage.tgz /path/to/mail okay? 

Thanks.