One of my kind readers made me aware of another e-mail tracing service, one that is called SpyPig.
As professional investigators we are highly familiar with ReadNotify, and use it regularly. We have introduced it into the local legal community, who is accepting it with open arms.
We have not yet tested SpyPig so we are unsure of how it compares to ReadNotify. At some time in the future we will compare it to ReadNotify.