It seems that your SMTP server is configured to send only emails with that have the same email address in the "From" field and that you used to authenticates into the SMTP Server.
This is done sometimes as an anti-spam solution.
You can try to solve by configuring the Qlik NPrinting "Default sender e-mail" with the same email address you used to authenticate. As alternative you can configure your SMTP server to allow that the "From" Address differs from the authenticated user.
To configure SMTP in Qlik NPrinting refer to Configure SMTP Server ‒ Qlik NPrinting