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.

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.