Apologies for the garbled message. Must've been a high ASCII character
in the clipboard....  :-}  Let's try again:
 
Versaterm-Link (part of Versaterm, a terrific Mac communications suite)
allows you to add any mail header you choose to your mail header. For
instance, I added the line "Resent-From: [log in to unmask] (Fox Mulder)" to
a test message.
 
I also found two two Eudora plug-ins for the Mac) which may help. They
are available from any Info-Mac site.
 
Calles Eudora Plugins 1.0 (comm/inet/mail/edr/calles-eudora-plugins-10.hqx)
Calles Eudora Sender Plugin let you edit the 'From:' field in the email
window.
 
Adam's Plug In 1.0.7 (comm/inet/mail/edr/eudora-adams-plug-in-107.hqx)
adds the following panels to your Eudora Settings: [...] X-Headers:
Lets you add your own X-Headers....
 
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  Peter C.S. Adams         http://www.geocities.com/RainForest/4725/
      PAGEMAKR listserv    http://www.hypercorp.com/gain/pm
      PHOTOSHP listserv    http://www.hypercorp.com/gain/pm/photoshp.html