Quoting Peter Bowyer:
> My big worry about this is that you're completely dependent on the
> behaviour of the other MTA - you've no logging, nothing to tweak,
> can't whitelist, just have to trust that the people sending you wanted
> mail are going to do the 'right thing'. That alone has kept me from
> trying it.
You can put these IPs on your usual MX and tell Exim to handle them
specially. So you can do with these connections whatever you like.