Exchange 2010 and callout settings

Hello,

I have just re-commissioned a development  Exchange 2010 SP3 server ready for upgrading and after checking the system, I have found that any incoming external emails are getting rejected by the UTM (9.4) with an address unknown error:

from="xxxxx@gmail.com" to="<user>@domain.co.uk" size="15694" reason="address_verification" extra="Address unknown"

The UTM SMTP settings haven't changed (as far as I known) and is set for "with callout"

The external domain is set to be routed by the UTM to the Exchange server via a Static Host list.

If I disable "with callout", it works ok.

I do also have Recipient Filtering enabled on the Exchange Server.

Internal email is working on the Exchange server - both to internal and eternal addresses.

Is there something obvious that would be stopping external emails coming in?

Thanks in advance.

Giles