Is emailing consumers without their consent illegal?

So I know technically it is illegal to send emails to consumers without their consent, aka, my title kind of does not make sense...but I was wondering how monitored is that really. So you can essentially email someone by getting their email from someone like a friend and then reach out to them...but they did not give me their consent, but that is never a big deal, because well who cares....now I assume there is legality issues when your spamming peoples emails 5,000+ at a time advertising something... my question is, is that something that could really get you in that much trouble because 99.5% of peole are just going to view it and either go Oh Cool or This is Spam and think nothing of it....I get confused about web scraping for emails based on niche posts that fit your niche products....I know your not suppose to spam random peoples emails, but is it monitored or taken that serious? It might be. I am just curious about the whole email scraping thing....some insight would be awesome...thanks mates

You can only get in trouble if you do it on a very big scale, no body will bother with you for sending couple of emails here and there..

awesome, thanks!!!!