Is it legal to crawl the web to get all (the worlds) techers email adresses, then email them?

Job ID: 35782213

Budget: $30 – $250 USD

We don't have a business yet, but we will soon.
We aren't selling our product, we are giving them it for free.
We plan to send one single mail, to everyone.
How will the laws effect this, dependent on country? Server location? Reciever adress, send address?
Can we use a program to feed in the schools domain (school.com), then fetch all the email addresses on that domain?
Other factors?
Related categories: Corporate Law Technology Law