[Exim] log_incoming_port

I am running a SMTP relay on a server I administer and would like to use
the log_incoming_port feature to track NATed connections. When enabled
however it does not seen to do the correct thing. For instance, if I send
a message on 34002 (found using netstat -a) the log shows the incoming
port as 53892.

I am sure I am doing something silly but if anybody could point me in the
right diretion I would be most grateful.

Thanks

Chris