Request on production stays pending, shows "No 'Access-Control-Allow-Origin' header" error after aborted

[Jan Van Echelpoel]

I built a microservice which handles a contact form request and sends the data to Mailgun:

index.js:

const { handleSendContactForm } = require('./send-contact-form/handler')

const sendContactFormCors = microCors({ allowMethods: ['OPTIONS', 'POST'] })

module.exports = router(
    post('/send-contact-form', rateLimit(sendContactFormCors(handleSendContactForm))),
    options('/send-contact-form', rateLimit(sendContactFormCors((req, res) => send(res, 200)))),
)

handler.js:

const RESPONSE_MESSAGES = {
    406: 'Invalid contact form data.',
    500: 'Something went wrong. Failed to send e-mail.',
    200: 'E-mail successfully sent.'
}

const handleSendContactForm = async (req, res) => {
    const data = await json(req)

    if (!isValidContactForm(data)) {
        const statusCode = 406
        send(res, statusCode, {
            statusCode,
            message: RESPONSE_MESSAGES[statusCode],
            errors: getContactFormValidationErrors(data)
        })
    } else {
        try {
            await sendContactFormEmail(data)
        } catch (err) {
            const statusCode = 500
            send(res, statusCode, {
                statusCode,
                message: RESPONSE_MESSAGES[statusCode],
            })
        }
        const statusCode = 200
        send(res, statusCode, {
            statusCode,
            message: RESPONSE_MESSAGES[statusCode]
        })
    }
}

module.exports = {
    handleSendContactForm,
}

This is working fine locally (without any cors issues), but when deployed to zeit now, the request status stays pending before aborting:

After aborting I get this error in my console:
Failed to load https://[REMOVED]/send-contact-form: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin '[REMOVED]' is therefore not allowed access. The response had HTTP status code 502. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

& in the response of the failed request:
{"status":"502","description":"An error occured with your deployment"}

but nothing shows up in the deployment logs.

This was working perfectly fine before, so I have no idea if this is an issue with my code, micro or the now deployment.

[Jan Van Echelpoel]

I was able to solve this. 🎉

In the sendContactFormEmail function I used nodemailer to send an e-mail with Mailgun's SMTP protocol but the request kept being pending and eventually aborted because the instance crashed.

Instead of nodemailer I used nodemailer-mailgun-transport and used Mailgun's API to send the e-mail which worked like expected.