Ruleset to lock domain to IP address

[James Fagan]

> We plan to introduce some premium filtering options for some domains. 
> This will result in all incoming mail to a given domain 
> arriving from a single known IP address.  To prevent 
> "back-dooring" we'd like to lock that in so any incoming mail 
> to a given domain from any other IP address is rejected or 
> dropped.  Can I create a ruleset to achieve that?
> 
> TIA
> Brian
> --

We do something similar, but we have it setup at the customers
firewall/router
to only accept connects on port 25 from one of our IPs (MailScanner
boxes). This
does stop the drive-by spam. We do this for all our clients permitted
they have the 
hardware to achive this. Not exactly as you want to do it, but its an
alternative. 
Besides most customers don't know anything about ports and routing so
you could charge 
them a maintence fee or something for comfiguring their
routers/firewall. 

Other than that I think you would be looking at some fancy pants
iptables.  

Or maybe there are other solutions?

James