Cloud Functions using Nodemailer, Email sent but never arrives - node.js

Question

I am using Google Cloud Functions to send E-Mail via Nodemailer, the mails are authenticated via Oauth2. It seems to work without any problems, the emails get sent and are also shown in the sent e-mails of my gmail account, but they actually never arrive at my other email address... Does someone know why?
This is my Code
const mailTransport = nodemailer.createTransport({
name: "gmail.com",
host: "smtp.gmail.com",
port: 465,
secure: true,
auth: {
type: "OAuth2",
user: gmailEmail,
pass: gmailPassword,
clientId: clientid,
clientSecret: clientsecret,
refreshToken: clientrefreshtoken,
accessToken: clientaccesstoken,
expires: 3599,
},
});
exports.submit = functions.https.onRequest((req, res) => {
const corsMiddleware = cors(corsOptions);
corsMiddleware(req, res, () => {
if (req.method !== "POST") {
return;
}
const mailOptions = {
from: req.body.email,
replyTo: req.body.email,
to: "myemail#gmx.at",
subject: `Absender: ${req.body.email}`,
html: req.body.message,
};
mailTransport.sendMail(mailOptions);
res.status(200).send({isEmailSend: true});
});
});
Thanks in Advance
Update!
Somehow i get some e-mails really late, but not all of them. More like 1 out of 10 and i also got this in my Mail inbox:
The recipient server did not accept our requests to connect.
Learn more at https://support.google.com/mail/answer/7720 [mx00.emig.gmx.net. 212.227.15.9: 421-gmx.net (mxgmx017)
Nemesis ESMTP Service not available 421-Service unavailable 421-Reject due to policy restrictions.
421 For explanation visit https://postmaster.gmx.net/en/error-messages?ip=209.85.222.194&c=sp ] [mx01.emig.gmx.net. 212.227.17.5: 421-gmx.net (mxgmx114)
Nemesis ESMTP Service not available 421-Service unavailable 421-Reject due to policy restrictions.
421 For explanation visit https://postmaster.gmx.net/en/error-messages?ip=209.85.222.196&c=sp ]

Answer 1

So after 2 days of trying i think the problem has nothing to do with my code....
Seems to be a problem with gmx servers, maybe i am on any blacklist? i dont know, account is new actually...
Nevermind sending it to my private email from my domain worked.

Answer 2

It is maybe not the reason of your problem, but you should wait that the asynchronous sendMail() execution is completed before sending back the response. If you don't do that you indicate to the Cloud Function platform that it can clean up the instance running your Cloud Function, without waiting for the asynchronous task to be completed.
exports.submit = functions.https.onRequest((req, res) => {
const corsMiddleware = cors(corsOptions);
corsMiddleware(req, res, () => {
if (req.method !== "POST") {
return;
}
const mailOptions = {
from: req.body.email,
replyTo: req.body.email,
to: "myemail#gmx.at",
subject: `Absender: ${req.body.email}`,
html: req.body.message,
};
mailTransport.sendMail(mailOptions)
.then(() => {
res.status(200).send({isEmailSend: true});
})
});
});