app.mailer.send callback not called

[mlistiak]

                        console.log(app.mailer.send);

			await new Promise(async (resolve, reject) => {
				app.mailer
					.send('email-plan-downgraded', {
						from: process.env.MAIL_FROM,
						to: accounts[0].email,
						subject: 'Platený program expiroval',
						upgradeLink: `${process.env.PROTOCOL}://${process.env.HOST}${port}/admin`,
						deactivatedOffers,
					}, (err, res) => {
						if (err) {
							console.log(res);
							reject(err);
						}
						console.log(res);
						resolve(res);
					});
			});

console returns only: [Function]

No error or response. Every help is appreciated.