smtp relay host with two mx entries

Hi misc,

i am trying to create a simple smtp client configuration, where the
client should only send local mails to a relay host. the key point is
that the relay host hides a redundant MX record with different
priorities in the DNS. A DNS A-record (or quad A) on the other hand does
not exist.

As I understand it, this is not possible with a relay statement, because
no MX lookup is done.

How must a corresponding configuration look like to be able to use a MX
lookup?

this is my (still broken) configuration:

<snip>
table aliases file:/etc/mail/aliases

listen on socket
listen on lo0

action "local_mail" mbox alias <aliases>
action "outbound" relay smtp+tls://<relay host with two mx entries>

match from local for local action "local_mail"
match from local for any action "outbound"
<snap>

Many thanks!