didn't find anything about this issue and maybe it's not possible, but:
is it possible to get a selection of relayhosts in subject to the
senderaddress?
To give an example:
mail from user "foo" goes over relayhost1.tld,
mail from user "bar" goes over relayhost2.tld
... and so on.
Is there a way to get this running with exim? If not, does anybody know
which mta ia able to do this?