Ben's response is correct.  I just had the same problem with a
subscriber's email looping.  Fortunately, some duplicate checking by
LISTSERV(R) ameliorated the problem.  Note the header:

|Received: from ccgate.tiffinu.edu (ccgate.tiffin.edu [199.218.239.3])

There is NO Internet  domain of TIFFINU.EDU, but there is of TIFFIN.EDU.
Check for subscribers in THAT domain.

/Pete
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