Exchange 2003 - 2010 Transition issue

I have installed a 2010 Server in an existing domain and have followed the instructions in http://www.petenetlive.com/KB/Article/0000234.htm which a number of people have referred to in previous posts.

I have a mail flow issue which is driving me nuts...

Mail flow to/from external email addresses is working perfectly for both users migrated to 2010 and those still on on 2003.

Internally, those on 2010 can email anyone on 2010 or on 2003, however those still on 2003 can only email others on 2003.

I have deleted and recreated the Routing Group connectors which seems to be the generic "do this for 2003<->2010 mail flow issues", but it's not changing a thing.

Any ideas?

+added+

2010 Server is SP1 + rollup 3.

2003 Server is fully up to date.