async nodemailer sendmail Promise

how can I get a ‘then’, or asynchronously return an API response?
I use fastify, but it doesn’t wait for a response if you make a callback inside.
I tried that, but the error: TypeError: a.then is not a function

         const a = await transporter.sendMail(mainOptions);
            a.then((error, result) => {
                if (error) return error
                reply.send({
                  messageId: result.messageId
                })
            })