What's creating X-X-Sender header?

[Patrick Wiseman <pwiseman@mindspring.com>]

My emails from a 'testing' machine include the following header:

X-X-Sender: pwiseman@mycroft

I'm 'pwiseman'; my machine's _local_ name is 'mycroft', but no-one in the
outside world needs to know that.  So what's telling them?  Exim?  How do
I stop it?

Patrick