SPAM could also be taken to mean a set of humorous backronyms, including: Short/Stupid/Silly Particularly/Pointless Annoying Messages, Self-Promotional Advertising Material, Self Propelled Automatic Mail, Send Post All Members and Sending Persistently Annoying Mail. SPAM is also a portmanteau of "spew" and "scam".
I have heard of Self Promotional Advertising Material being the most accepted explanantion apart from the Monty Python sketch.
The point is that ,just as with spam in the sketch, whatever else you want to receive in your mailbox you get unsolicited e-mail 'spam' with it whether you ask for it, or want it, or not.Everything comes with it !
In the early days of the Internet, some mindless people would send out junk e-mails filled with the word 'spam' - linked to the Python sketch where the word was repeated over and over again. The idea was to fill the screen so that previous responses would be unreadable, as they would move up the screen and out of sight of new viewers.
It has since passed into common use as a catch-all for any unwanted e-mails.