@eeeee NodeBB can technically send mail straight out of the box, although that's obviously not going to be really good deliverability-wise because your IP is unknown (or worse) and you're not sending any authentication-style headers, both of which are baseline assumptions that need to be met otherwise your emails just end up in the spam bin.
We use the SendGrid plugin here, with its generous free tier. NodeBB doesn't send too many emails, so that should be ok to get started.