Thanks for that.
The curious thing is that no other businesses that we are e-mailing with has caused us any problems.
Having exhausted our own (very limited) expertise we turned to our IT Support Company - and they too couldn't sort it out. The best suggestion we had is to pay for a Static IP address for our e-mails. But then another advisor suggested this may also not be immune to organisations such as SORBS blocking us.
The VPN idea sound interesting. I will explore that with said IT Support guys.
Thanks again