How to store an arbitrary object in a Mail using ArbitrarySerializable?

I'm in the process of converting code for James 2.X to 3.8.

I'm having some trouble storing an instance of an arbitrary class as a Mail
attribute.

James 2.X supported storing Serializable objects directly as an Mail
attribute. In James 3.X this was changed to a specialized procedure using
JSON serialization.

It looks like I need to implement ArbitrarySerializable to support storing
arbitrary objects. There is example implementation of ArbitrarySerializable
but that's only a simple example supporting an Integer which works because
AttributeValue.of already supports Integer's.

Can someone help me with an example on how to store complex objects as a
Mail attribute?

Kind regards,

Martijn Brinkers