[Exim] name resolution?

I have a linux network running with a spoof domain name, and am drawing mail
down for a real world domain.

On my local domain users are 'firstname_lastname' format @spoof.domain and the
real world users are 'firstname.lastname' format @ realworld.domain

Having drawn down the email, all are punted to postmaster/root with the
following warning:
'X-Fetchmail-Warning: recipient address 'firstname.lastname@???
didn't match any local name'.

Using command line 'mail' all permutations, 'firstname./_lastname', are
accepted. So where am i going wrong ?????

All help gratefully accepted.

Thanking you all in advance.

--
PCs are like air conditioners,
Open windows and they stop working............