[exim] Outgoing IP address

Dear all,

my Exim server runs on a virtual machine that has 2 IP addresses
configured. Until today, I did not have any special configuration to
specify which address should be used for outgoing connections. Exim (or
the OS?) always used the "first" IP address (the first one in ifconfig).

Today I did an upgrade at the VM _host_ from Debian Lenny to Squeeze,
i.e. I did not upgrade anything in the virtual machine where Exim runs.
Then suddenly, Exim started to use both IP addresses for outgoing
connections. I could not figure out when it would use which one.

I temporarily fixed this by adding the interface option to the
remote_smtp transport. Nevertheless, I'd like to understand what was
going on there. Does anybody have an explanation?

Thank you!

Greetings,
Peter