Configure direct-delivery incoming email for self-hosted sites with Mail-Receiver

It appears there is an assumption that the mail-receiver will always be the same domain as the base forum. When that is not the case, how do we setup TLS?

For example:
forum => forum.domain.tld
mail-receiver => mail.domain.tld

In mail-receiver.yml, the TLS points to the base forum certs. Is there a way to have mail-receiver to obtain thier own certs?