Nodemailer is sending emails with the same subject as thread - node.js

Question

Nodemailer is sending emails as a thread if the email has the same subject as a previously sent email. But I want to send the emails as separate emails, even if they have the same subject as each other.
I have an application that sends notification emails to users. The emails all have the same subject: notification. This is causing the emails to show as a thread, at least in Gmail:
How can I make each notification email send separately?
const nodemailer = require('nodemailer');
const logger = require('./logger');
class Email {
constructor(email, to, pass) {
this.user = email;
this.to = to;
this.pass = pass;
}
get mailOptions() {
return {
from: this.user,
to: this.to,
subject: 'notification',
html: 'You received a new purchase in your shop.',
};
}
get transporter() {
const transporter = nodemailer.createTransport({
// for zoho emails
host: 'smtp.zoho.com',
port: 587,
secure: false,
auth: {
user: this.user,
pass: this.pass,
},
});
return transporter;
}
send(cb) {
this.transporter.sendMail(
this.mailOptions,
cb ||
((error, info) => {
if (error) console.log(error);
if (info) logger.log('silly', `message sent: ${info.messageId}`);
this.transporter.close();
})
);
}
sendSync() {
return new Promise((res, rej) =>
this.transporter.sendMail(this.mailOptions, (error, info) => {
if (error) rej(error);
else res(info);
this.transporter.close();
})
);
}
}
const email = new Email(
'somesendingemail#gmail.com',
'somereceivingemail#gmail.com',
'SendINgEmAIlPassWoRD'
);
email.send();