When I try to import a function with npm modules that I exported, I get "Class extends value undefined is not a constructor or null" in sveltekit

I am trying to send emails with values in my form with sveltekit using npm module: "nodemailer", but when I import the function I have I get this error: "Class extends value undefined is not a constructor or null"

This is the mail.js with the function:

import nodemailer from "nodemailer"

var transporter = nodemailer.createTransport({
    service: 'gmail',
    auth: {
        user: 'mymail',
        pass: 'mypass'
    }
});

export function sendEmail(text) {
    const mailOptions = {
        from: "from",
        to: "to",
        subject: "test",
        text
    }

    transporter.sendMail(mailOptions)
}

and this is my +page.svelte in /contact folder:

<script>
    import Navbar from "../Navbar.svelte";
    import Footer from "../Footer.svelte";

    import {sendEmail} from "./mail.js";

    //sendEmail("message")
</script>

<Navbar/>

<form id="contact-form">
    <textarea id="message" cols="30" rows="10"></textarea>
    <button type="submit">Send</button>
</form>

<Footer/>

The weird stuff happens, when I go on the page, I get 500 error with the description I provided, but when the sendEmail() function is uncommented, it actually sends the email on mine.

I want to send my mail with the values from the form.

So after digging deep into the answer provided by Tobi Obeck, I finally got it right. I still don't know why the script has been executed, but I do understand why the page hit 500 server error everytime I tried to post the form. I am sending my code for everyone that is out there, here's my solution:

import nodemailer from "nodemailer"

var transporter = nodemailer.createTransport( {
    service: "gmail",
    auth: {
        user: "mymail",
        pass: "mypass"
    }
})

/** @type {import('./$types').Actions} */
export const actions = {
default: async ({ request }) => {
    const data = await request.formData();

    const email = data.get("email")
    const message = data.get("message")

    const mailOptions = {
        from: email,
        to: "tomail",
        subject: `subject`,
        text: message
    }

    await transporter.sendMail(mailOptions)

    }
}