You can browse all the properties in the javadocs for the javax.mail.internet package.
There are some that you can set that might help with this. But at some point the message
will be so broken that there's nothing reasonable you can do with it.
It's possible to supply your own class that will "clean up" the Content-Type header
before it's used, so if you think you know how to do this better than JavaMail, you can
write your own code to do so.
But you'll need a recent version of JavaMail to use all these features.
It is, of course, much better to fix the mailer that's sending this garbage.
That's the real source of the problem, not JavaMail.