Java is backwards-compatible - using classes compiled with (or for) an older Java version is not a problem. It seems that you are not adding the jar file properly to the classpath (or the jar file is damaged - try if "jar tf mail.jar" lists all its classes).
Put the moon back where you found it! We need it for tides and poetry and stuff. Like this tiny ad:
how do I do my own kindle-like thing - without amazon