I've come across this before, and I don't know if the mechanism in my case
might be the same as in yours.
 
I use a Pine mailer, and it does some magical things that I'm not aware of
and that in some instances are "overly smart".  (If your mailer acts
similarly, then this may be the problem.)   Sometimes I'm sent a file
which I believe to be composed solely of low-order ascii characters, but in
fact there are stray high-order characters or tabs or what-not, but
they're no apparent when viewed through the Pine mailer.
 
When I try to PUT such a file to Listserv, Pine detects the high-order
characters and before transmission, alters the entire file, specifically
by tansforming any instance of "=" to "=3D" (and several other
tranformations).  Since the PUT cpmmand always requires a PW=XXXXXXXX
command, Pine changes this to PW=3DXXXXXXXX and thus when Listserv
receives the file, it rejects the PUT because the password *it* sees is
indeed incorrect.
 
The solution for me is to make doubly sure that all high-order ascii
characters are stripped away from any file I send to Listserv via the Pine
mailer.  (These error-laden files are often the result of trying to create
an ascii file from a word-processor document.)  If you have a "smart"
mailer like Pine, it might be doing these same things to files -- and only
those files -- that contain high-order ascii characters.
 
-- Roger Burns   [log in to unmask]