Slightly different but I bought a brand new O2 contract phone a few weeks back through mobiles.co.uk
Received the phone on Tuesday at 9am. At 11am I started receiving mysterious calls - went to whocallsme.com to check the number and found them out to be companies selling mobile phone insurance - I'd only had the phone 2 hours and was livid that O2 had appeared to "sell" my number so quickly
Turns out that these companies auto dial thousands of new numbers (all new O2 mobiles have the same 07 prefix) so they set the diallers to dial random sequential numbers til they get a ringtone and once you answer it's put through to a sales operative. The number is repeatedly called, much to the annoyance of the owner. I answered it after owning for three hours and made a complaint, they never called me back!

My point is, it wasn't actually anything to do with O2 at all.
It's my understanding that the function for spam emails is similar, bots auto-generate emails at popular free email services, like hotmail, yahoo & gmail til they find a real one. Once they do it's stored and repeatedly mailed to.
I would very much guess that you were caught out this way rather than through Toyota selling your email to an online pharmaceutical company specialising in erectyle dysfunction problems. Unless, like you say, the biggest car maker in the world has had their systems hacked....
Obviously if you feel that Toyota have gone against their own policy you are better off raising it with them directly.