True company ratings

Well right off the bat you have a bit of a problem here.
There's only one way to truly verify that somebody works for somebody else. Talk to the owner. How many owners are going to allow some 3rd party to verify who's worked for them?
Will you verify that Joe Bob worked for me because he shows you a paypal payment and note where I paid him $5 for an article he wrote and told him if he tries to blackmail me again I will have him reported to the authorities, since his article was half the required length and on the wrong topic?
You see what I mean?

That's too bad you had an experience you didn't like... However, it's my experience (even when I was the employee) that employers have every right to let a person go, and when that happens, that person is going to be mad, no matter what. Work is our lifeline and how we survive. Society has raised us to have 1 or maybe 2 jobs that account for our money to pay the bills and so on.

You cannot remain logical when something threatens your ability to put food on your table, feed your kids, pay your bills and not live on the streets. It doesn't matter if the true threat is yourself, and your lack of ability to perform your job as requested.

I can tell you I would not support such a website, generally the people who used to work for a company, are the people who were not compatible with that company. If the worker and the company are not compatible it's time to move on. Don't get angry about it, learn from it.

Websites like ripoffreport.com make me sick. They are filled with lies, garbage, crap, and angry people that are just going to be angry no matter what! If you have not ever come across and employee, a customer, a partner, a boss, who rubbed you the wrong way, then you must be very young and very new. This is life, and the challenge we all face. Learning to get along, and finding our happy place. Yelling about what didn't work on some website won't change anything. Try doing something useful instead.
Take, for example, they have some survey or list of the top best employers of the world. Make your site cater towards that, promote that, and provide a forum for people to discuss things, but discourage flaming and complaining it doesn't accomplish anything.

I don't think your idea is feasible. Unless you target only BIG companies that have their own domain and email set up for every employee. What if there is no email for every employee...?

On the other hand, many companies forbid using the company's email address for private purposes. Furthermore, if the employer doesn't like the comment of one of his employees, he can track down who posted it... and can fire the person: broke the company's rule of not using email for private purposes. Obviously, posting comments about the workplace on a third party site is not in their job description.

I'd say forget the email verification. It just doesn't work.