Hi,
thanks for all you help on this, I've found the answer which is very bizarre as I've never come across it before.
for anyone connecting to an exchange account using RPC you have to add two records into your external DNS, the A record relates to the certificate that is installed on the server, i.e. remote.domain.com. The second is an SRV record, which allows auto discover to re-route to a certified domain (At least this is how i understand it)
here is an example of an SRV record if you're interested or don't know what it's about (as i didn't)
_autodiscover._tcp.company
.net. SRV 0 0 443 remote.company.net.
And here is where i came across the resolution, under heading "Remote Clients and Windows Mobile 6.1 devices"
http://blogs.technet.com/sbs/archive/2008/12/03/how-outlook-2007-and-windows-mobile-6-1-use-autodiscover-with-sbs-2008.aspxAgain thanks for your comments, but this is how i resolved the issue.