It works, but the problem is it works too well.
We found several bugs with this product, and as since have disabled it on our forum.
#1 - When receiving an email, it always stated to the user "New Conversation Started", even if it was a reply.
#2 - If a user has not visited the forum to check an email notification, and someone else sends them a message, they do not get notified of any other forum activity (by email), until they visit the forum again.
The developer has advised that this is by design, and that the product is actually a legacy product from the Xenforo 1.x days, and ported over because it was easy to do.
For some forum owners it might work despite its limitations, but it's not for us and we won't be renewing our license for this addon.