[exim] @mx_any behind NAT...

I am trying to set up a backup MX server for many domains. I need to
not have to maintain a separate list of domains in the server config, so
I'd like it to relay based on MX.

In my config I have...

#
primary_hostname = mx2.dotcomimaging.com
#
domainlist relay_to_domains = @mx_any
#

...and in my ACL I have...

#
accept  domains       = +relay_to_domains
          endpass
          verify        = recipient
#

I am testing with the domain dotcomimaging.com, which has
"mx2.dotcomimaging.com" as the secondary MX record, but messages are
always getting rejected. So, I have two questions...

1. Is there a way I can test specifically why it is getting rejected?

2. Could it be that because the exim server is behind a NAT, although
the hostname mx2.dotcomimaging.com will be in the MX list, the private
IP address 192.168.100.116 will not?

Any help much appreciated.

Regards, Adam Bown.