How can I make LEM take any notice of hostname field in syslog messages

We need to forward syslog messages from several systems via a syslog relay to LEM.

This works but LEM identifies all events as relating to a node corresponding to the relay server's IP.

How can we make LEM use the hostname in the syslog message, to identify the originating system? We know the hostname is there and correct in syslog messages to LEM because we can see if we substitute a default syslog server for LEM.

Thanks for any help you can give