I am bored, and therefore I intend to display my boredom on the internet.
Whilst looking through my spam folder (I really was that bored), I noticed that there were a bunch of messages at the top which had been there since I last looked (a long while ago).
At first, I dismissed it as my spam filter not working, but upon closer inspection, it appears that they were sent in October...
October 2018, that is. 4 years from now.
I therefore conclude that a type of spam designed to bug you to open it is what I have dubbed 'floating spam'. That is, spam which was sent to you from the future. It will appear in your inbox if you perhaps run a small business, and are using a cheap mailserver to server you mail without a built in spam-filter and you only access your mail through webmail. If it's all poorly put together, you will theoretically end up with these spam messages stuck at the top, hiding your other mail.
Of course, for me, the fact it was sent 4 years from now indicates to my spam filter that I am either involved in some Doctor Who plotline involving complicated timey-wimey stuff and email, or I have been sent some spam. It's pretty obvious which one it is.
BE WARNED! FLOATING SPAM IS AMONG US!