Bizarre InvalidClassException

Question

I'm getting an InvalidClassException reading back an object which was written 5 seconds ago. I know why this exception occurs in general, however I can't understand how it could have occurred in this case.
The class causing the problem is javax.mail.internet.InternetAddress, which is not Serializable, hence the problem. (Most annoyingly, it seems like this class is a really, really simple class, so simple it's amazing they haven't made it Serializable already.
The thing that makes this bizarre is, I have checked every mail.jar on my system (the system consists of 3-5 different components which run in different JVMs) and they all check out (using diff) to be the same file, and I can't find any stray InternetAddress.class files which might also cause the problem. I would even symlink them to prove the point, but unfortunately certain things break trying to read symlinked jar files.
Yet, the mismatched serialVersionIDs are always the same two, one starting with 7 and one starting with 9. It would be really nifty if there were a way to 'find' a class file on the local machine with a given serialVersionID, then I could do an exhaustive search in every jar and zip on my system to find the offending class.

Answer 1

http://java.sun.com/j2ee/sdk_1.3/techdocs/api/index.html
I gather it would have an 'all implemented interfaces' part if it were.
Still, we're using MarshalledObject to hide away things that aren't supposed to be serialisable, since we're (supposed to be) using the same version of all the classes for a particular installation of the system.
But like I was saying, all the jar files I have are the same file (same length, same contents), and I checked with serialver, they are all the -7...L one, so version 1.3.) This is what makes me suspect that someone somewhere has lumped InternetAddress inside another jar file, and I am currently scanning my entire filesystem for all jars and zips so I can see if anything other than mail.jars contains this class.