< #5.5.0 Smtp;554 Sorry, Message Looks Like Spam To Me> What Is The 3.5, 5.0,ect,ect, Players?

What is the 3.5, 5.0,ect,ect, players? - < #5.5.0 smtp;554 sorry, message looks like spam to me>

I've heard, everyone speaks ect ect Player 5.0 or 3.5 or 4.5 player player, wel wat that means and how comest thou?