Farewell emails

Well…I have seen a lot of these mails and it makes me wonder whether a person who leaves an organisation actually means what he writes in his last mail.All those sugary words quoting how great it was working here and what a great support everyone was,blah blah..Do they really mean it? If they do mean it,then why do they leave?Of course some have genuine reasons,what abt the majority of them? “Keep in touch and mail me at xyz.com” Do people actually contact them?On the contrary,I have seen some nasty mails with names and rude remarks.It must have been a really bad experience for them.Its important to give a correct and a sincere feedback but it must be done in a proper,professional manner.