[exim] Smarthost on Specified Interface

Exim4 on Debian Linux. Connecting over ppp links, I have :

queue_domains = ! +local_domains

/etc/ppp/if-up includes the line:

/usr/sbin/exim4 -qf

To run the queue when the interface comes up.

This has been working fine for a while. But now I am toying with a new
ISP. When connecting to the old ISP (isp1), I want to run the queue,
when connecting to the new one (isp2), at the moment I don't. Because if
exim tries to send mail with return address care of isp1 via isp2, the
mail will, of course, be bounced by isp2. I'd like to have the
possibility of isp1 And isp2 being up at the same time. Which I've got
the modems and serial ports for.

ip-up distinguishes whether to run the queue or not according to the IP
range assigned by ISPs. But the thing is that when isp1 comes up, ip-up
will run 'exim4 -qf' as planned BUT exim4 could then in fact use
interface to isp2 (if it's up at the time), where I would only want it
to use the direct interface to isp1.

I see that there is the configuration item "interface=", but I can't
specify the interface in a configuration file in advance, as they are
dynamically assigned (dhcp) when the interface comes up. I've set 'route
add -net x.x.x.x netmask dev pppx", in ip-up, but this doesn't seem to
guarantee a result.

Anyone know if there is a command line option to exim4 that will have
smtp sent via isp1 interface or fail? I've looked, but cannot see.


regards, TimJ.