Sending e-mails on the Internet is no longer a trivial matter. It used to be a wild west where anyone could send e-mails using any “from” address. Spammers took advantage of this.
To cut down in the amount of incoming spam for their users many e-mail servers now inspect incoming messages and use various techniques to figure out if the e-mail is valid or is spam.
From the screen shots I see in this forum thread it appears NodeBB was created during the wild west days. Much of what they are suggesting won’t work in today’s Internet.
While @PeterJones suggested using any major provider I suspect you will run into sending limits with most of the providers. NodeBB will be firing off batches of nearly identical e-mails to many people to alert them about updates to forum threads and/or to send them daily digests. Those batches will look like spam attempts to most of the major providers which will shut you down.
I noticed that notepad-plus-plus.org has an MX record but there’s either no inbound mail server configured or it rejected my attempt to connect to it. I’ll assume the MX record is an artifact from the hosting company and that @donho did not buy the e-mail hosting part of the package.
As I noted earlier, sending e-mails on the Internet is no longer a trivial matter. Many hosting companies don’t allow their customers to run e-mail based forums as the resulting traffic can cause the hosting company’s IP network blocks to get blacklisted. I started to look into if the company that hosts the notepad-plus-plus.org web site, https://www.hostinger.fr/ , allows e-mail forums but I don’t know French. I tried viewing the hosting site via the Google translator which worked well until the site blocked me as they did not want me to view their site via another site (google’s translator).