Plugin: Emailer (Local) Smtp SSL Vs nodebb-plugin-emailer-mailgun
Unless you are only sending email to a local email server, I would recommend against using a local emailer service. There are many benefits to using a transactional service like mailgun and many caveats to using a local emailer. The link has many examples.
The number one reason I would chose mailgun is deliverability. Many large email providers (gmail etc..) are very stingy about accepting email from non-verified or non-whitelisted senders. And if it even decides to accept the message, it will give it a huge bias toward marking it as spam. Using mailgun or another service is no guarantee, but it gives you a much better chance that your emails will be received and kept out of spam.
That said, many people use emailer-local and have never had such problems, but I say, "Why take a chance?"