Here's my report about treating the output from:

spamassassin --mbox alltests.txt >scanned.mbox

as an mbox file.

In summary
----------
If you use SA to process a multi-message file with the --mbox option the
output is an acceptable mbox file.

Detail
------
After passing my standard regression test case through Spamassassin the
result was successfully processed by my JavaMail/mstor based
application. alltests.txt is an mbox format file containing 12 messages
that illustrate a variety of test cases for my application.

After this I ran my usual application stress test, which contains some
25,000 messages.

SA processed these without any problems. The result ran through
my repair program successfully. This replaces "undisclosed-recipients:"
with a default e-mail address and also any ';' separators in a
recipients list by ','. Both of these upset JavaMail, which has rather
picky header parsing, hence the repair program. Following this the file
was successfully run through the application.

====

Many thanks to all who helped me with advice.

Martin