I don't expect blizzard to tailor emails to each individual based one what they have already purchased as it's a general newsletter.

Except it isn't. It wasn't a "Here's a general overview of what's going on in Hearthstone", it was pretty much a single line email saying "This product is out. Buy it!" being sent to people that had already bought it.

They don't need to tailor emails in the "Get intro A if they have spent over 46.7 minutes in a Tavern Brawl, or get intro B if they spent more time in Arena. Use paragraph 1A if X, use paragraph 1B if Y, etc.

I don't want to have my emails tailored to me, I just don't want to receive an advert for something I've already purchased.

And if it's so hard to do, how does basically every company manage it? I kept getting emails to purchase GTAV from Rockstar (since its release), and when I actually finally did (earlier this year - was waiting for when I finally got a PS4), i didn't receive a single further email about it, for example (yet still receive emails about their other games).

It is possible, and othet companies on Blizzard's level (or even smaller) implement it well.

If your point is "Well, they just add all emails to a mailing list", that's the issue people have - Blizzard haven't updated their email system since the late 90s. I'll agree, if thus was 1998, I wouldn't expect more.

But it's not and other companies have shown it is possible to do.