[Exim] Rewriting sender headers written by Netscape

Hi,

how do I rewrite a Sender:-header not containing a domain, just a
username?

In my case, Netscape's Linux version inserts "Sender: be". Usually, the
final MTA appends his (!) domain to it, so it's possible for the
recipient to have a "Sender: be@???". Of course, this
email doesn't exist...