[Exim] odd "from" header

Hi,

I'm having a problem with Exim 4.14 in that mail that it receives and
forwards has the from header set to mail@<the_host>.

This is connecting via the command line rather than through another MTA
until it is working properly. Searching for "mail" and "from header" is
producing rather a lot of non relevant results, as you can imagine.

Dan