Are you unsure if an email invitation is from a predatory publisher or a legitimate one?
Jeffrey Beall, a librarian at the University of Colorado Denver, had compiled lists of "potential, possible, or probable predatory" journals and publishers. As of January 2017 the list is no longer updated. Details on its cessation can be read at Retraction Watch.
An anonymous individual has taken on the responsibility of preserving Beall’s list and updating it when potentially new predatory journals and publishers are identified. Find it here:
Note: These lists are by no means comprehensive and authoritative. They are designed to help authors, but are not the final authority on this topic. The lists are frequently updated, but all authors should conduct their own due diligence to reach their own conclusions (see tabs on evaluation and quality indicators).