Okay, thanks @julian
This isn't ideal considering future updates, but it is a viable solution.
[nodebb-plugin-emailer-mailjet] Mailjet Integration
This plugin allows NodeBB to use the Mailjet service to send emails. This plugin is adapted from mailgun plugin by @julian
npm install nodebb-plugin-emailer-mailjet
Activate the plugin in the admin panel and set up the API Key & Secret Key. You need to setup the domain or the email address from which you send emails on https://app.mailjet.com/account/sender
Plugin updated for NodeBB v1.x.x. Also updated to use the latest Mailjet API.