[exim] Exim server behind NAT router (and HELO)

I'm running an Exim (version 4.43 on Linux) server on a private network
with an address-translating router. Works rather well, but the address
translation means that the HELO (or maybe it's EHLO? It really makes no
difference in this context) won't be valid - so if I e.g. send to the
Exim router at work which does HELO checks, I get a HELO warning. That's
if I don't use hardcoded addresses or anything in the Exim conf, of
course. I can't just insert the info there, either, as the router has
dynamic address setup, so the address it translates to as well as the
associated hostname might change.

Is anyone else running Exim in such a setup? What's the best way to get
a correct HELO?

- Toralf