[Exim] Help with acceping underscore in FROM:

Greetings. Does anyone know how I can force Exim to accept an address of
the form:

FROM: test@test_host.domain.com.

Currently exim will give a:

501 test@test_host.domain.com: malformed address: _host.domain.com may not
follow test@test

I know the underscore isn't valid but I really need to accept mail from this
particular address.

Thanks!

Jeff