Are you using an address from the same domain as the used in the SMTP settings? If not, I have seen this be an issue where the SMTP server won't allow emails coming from other domains. For example. I cannot use my Gmail account info as the sender if my SMTP settings are configured for a Qlik domain.
Another possibility is that you need to give NPrinting access to send the emails. If you are using something like Gmail, you need to go into your account settings->sign in & security->connected apps and sites and then allow less secure apps. See the screenshot below: