Weird JavaMail problem

I've got the strangest problem with some Java code that uses JavaMail to
access 
an external (NT) mail server (without authentication). On our old
production 
system (V4R4), with a SOCKS proxy between it and the mail server,
the code 
works fine. But if I run the exact same CLASS file, with the exact
same 
"mail.jar" and "activation.jar," on our new production box (V4R5), it
gets a 
null pointer exception in the javax/mail/Transport.send0 method.
Looking at the 
source code for the Transport class (downloaded from Sun)
hasn't given me any 
ideas, except to tell me that "send0" is a private
method that is called from 
"send," and does all the "dirty work"
thereof.

--
J.Lampert