Sendmail smarthost relay to MX instead of A record

Hi all,

I have some older RH4 nodes that rely on sendmail for delivery. We have now a requirement to use as a smarthost a specific filtering system that has built in redundancy via two MX entries found under a filtering.domain.tld form.

So if I do a dix mx filtering.domain.tld I can see the two nodes responsible for receiving and filtering data.

From my initial testing it seems that putting in the config this routine:
define(SMART_HOST',filtering.domain.tld')

will fail delivery as it tries to do an A record search and this is not matched as it's not provisioned in DNS (we only have MX entries for this domain and A records for the two filtering servers)

Anyone knows if this can be done? The only idea that I have is to put the MX nodes under the SMART_HOST like `server1:server2' but this makes content static and if something will change in the future in regards to MX assignment I will have to reconfigure all the servers again.

PS: because we also have some postfix nodes on newer systems, the relayhost found here behaves like this out of the box: It first searches for MX and if that is not there for A. I'm thus looking for an efficient way to do this on sendmail (only once hopefully).

Thank you,
/MariusG