[exim] exim -bh and $interface_port

Exim version is 4.34.

I was a bit surprised, when trying a new configuration from a
simulated exim -bh run, to find that the $interface_port wasn't 25. In
fact, our new ACL tests for all three of the port numbers that we're
supporting (25, 587 or 465) - but the ACL, as written, was concluding
that it wasn't /any/ of those.

My colleague said that's easy - you can specify the interface port
after the IP address on exim -bh. So I did. But the $interface_port
still wasn't 25 in the ACL.

I printed it out and found that it was -1 (which happens to be the
same as for a locally-generated mail).

I've had a reasonable hunt around in the documentation and the
mail archive, but I don't find a mention of this, so I thought I'd
mention it to the list.

thanks