From reading postfix.org and taking config settings from other servers that do work. The biggest question is why does it work to some email addresses and not others?
First thought was that the destination ISP has a whitelist which would explain why no BT emails get through at all. However the logs indicate that postfix isn't getting as far as contacting their mail servers to even be told there is a block.
I think it isn't anything to do with mysql as I have dumped the email alias table to text as key/value pairs and built it as hash table, which made no difference.