Your answer was in the info you gave....... she is coming in with a
From field of:

[log in to unmask]

yet her return path (or default reply addres sor just Reply to
field) is shown as:

Return-Path: <[log in to unmask]>

if you check her subscription I bet you she is down as the
Return-Path one......

most likely causes in order are:-
1. malconfiguration of mail application....
     see if she has the ability to set her From field in her mail
     application, looks like the mail server or gateway is setting
     it for her which means the mailer she is using is sending it out
     blank.

2. changes to mail aliasing.... she'd have to check with her sys
    admins to see if anything like this took place...... (it has
    happened before in companys where they dont realise that  a
    change in aliasing will effectively veto any lists their users
     are subscribed to...

3. changes to mail server such as upgrades, moves etc..... especially
    ones that use some sort of semi-intelligent addressing tables...
    once again she would have to talk to her sys admins to find out
     if anything like this occured....

AFAIK Listserv only looks at the From: field for verification of the
subscriber....... out of the above three, number 1seems the
likeliest cause/solution because of addition of the word POSTOFFICE
to the e-mail address...

Debra
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Date:          Thu, 8 Apr 1999 21:54:25 -0400
From:          Mata Giri <[log in to unmask]>

-----snipo---------

You  are  not  authorized  to  send   mail  to  the  DARSHAN list
from  your [log in to unmask] account.

-----snippo---------

Return-Path: <[log in to unmask]>

-----snippo---------