There are a lot of suppliers of free SMTP servers online. I would to suggest you to start by doing an online search with Google.
Google ifself supply a free SMTP server for Gmail users. More information on the official online help of Google https://support.google.com/a/answer/176600
Please select the service that you prefer. As Qlik we are not affiliate with any of them.
If you select Google remember to deactivate the two steps authentication and to enable the less secure apps option Account settings: Your browser is not supported. Other SMTP suppliers could have the two step authentication issue.
In the Qlik NPrinting scheduler log file nprinting_scheduler.log you will find logs about reports sent via SMTP.