Is there a website that lists the journals that are actually scams? I constantly receive emails from editors asking me to submit papers for their journals. 

However, only a handful of them originate from a trustworthy source. The vast majority are just scams — journals with no reputation at all, willing to publish an(y) article for a fee.

My biggest problem with that is that I lost one or two real good opportunities to publish because I dismissed the email, thinking it was a scam.

Is there a list that I can use to easily check the reputation of a journal, in order to avoid this to happen?
There used to be one, called Beall's list, which listed predatory journals. The author however took it offline, apparently partly because of legal threats. If you search online you will be able to find archived copies of the list, but as far as I know these are not updated any more. You can read about it here for example.