There is no way of doing this with the default mail-behaviour. You can extend the existing behavior and roll your own implementation, that decides per-mail what domain to use. Since this is not a common use case, this won't be available in next versions of activiti.
If you really don't want to customize the email-task, you could use an SMTP-proxy, which decides which actual host to use, depending on the e-mail that's being sent
