I have the same issue. When you try to signup for the Newsletter it comes back with “Error during sending: please try again.”
I haven’t even gotten as far as trying to send a newsletter yet!
I only installed this plugin recently, tested it at the time and the signup part was working, but now it isn’t.
I have also updated to WordPress 4.3 so I don’t know if that is part of the problem.
I have tried uninstalling and reinstalling (interestingly though when I did this all the settings that I had entered were still there so I’m not sure if this actually worked properly or not). I have also tried deactivating and reactivating as per the advice above, but so far no luck.
Any ideas anyone. From the write-up on this plugin, it is exactly what I’m looking for but if my clients can’t even signup for the newsletter then what’s the point? Please advise asap or I’m going to have to move on and find something else ??