There should be no difficulty at all in doing this. If they dial into your network then they use your outgoing mail relay server, and yours alone. Period. (Unless you have some kind of agreement in a roaming system where you authenticate your own users to someone else's dial-up and vice versa, in which case you only allow the user to connect to the the "home" ISP's mail relay host(s).)
Or for those who complain that "reconfiguration" to use the local mail relay is too difficult, howabout setting aside a /24 from somewhere (the swamp ?) and writing an RFC that says "thou shall not route this" ala private address space, but if you wish to offer "local" services, then the user only need caryy one "well known" address range around with them. Give it some well know DNS: "mail.local." :) and so on. (OK New TLD required). This could also fix deciding which DNS servers to use amongst other things. Peter -- Peter Galbavy @ Home in Wonderland http://www.wonderland.org/ http://www.whirl-y-gig.org.uk/ http://www.demon.net Be remembered not for your final destination, but for your journey.