[exim] Exim4 on Debian cannot receive mail

Greetings,
I'm fairly new to Exim but had it working well on a CentOS (rpm
version)that was a VPS with 'real' IP.
Now trying to get it working on Debian with a valid (static) IP in a lab
environment at work. IP of 64.x.x.x is natted through a Cisco Pix to a
10.x.x.x. Can send mail fine but get this bounce to outside mail client
when attempting to receive mail;

'550 relay not permitted (in reply to RCPT TO command)'

Does this possibly have something to do with the nat address? Do I need to
add the 10.x.x.x address somewhere in the config file to allow this.
Domain resolves to 64.x.x.x if that matters.

thanks in advance for any info,

Chas.

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