Occasionally we get this message on our 1.8c copy of listserv on a VM/CMS system.  Just figured out why.  Perhaps this is fixed in 1.8d

The mail had header errors and was not stored on the server.  However, it was sent on the editor for approval. When the editor sent back the OK, the server couldn't find the original since it had never saved it.
Here's a copy of the log that tracks the problem:

Mar 1998 02:09:26 Processing file 1898 from [log in to unmask]
     (LSV82I) <E> "From:"/"Sender:" field is missing.
Mar 1998 02:09:30 Requesting confirmation, cookie=1FF610, ID=3347A3
Mar 1998 02:09:30 Sent information mail to [log in to unmask]

Turns out the mail had an blank line within the headers which made the Date, From, To and Subject appear to be in the body.