nodemailer multiple email sending using SMTP causes connection close errors - smtp

Question

I am having problems when I do a looped call to Mailer send email function.I am testing sending of over 50 emails and the most number of successfully sent were 30 so far. The rest is stopped by the following errors. I redacted the complete email address for the robot for privacy reasons.
[2018-09-03 05:18:32] INFO [0rULO04aig] User "xxx.robot#xxxxxx.com"
authenticated
[2018-09-03 05:18:32] INFO [#1] Sending message
<3299d22a-e0c8-00b8-4f7d-4d0455f85b31#xxxxx.com> using #1 to
[2018-09-03 05:18:32] DEBUG [0rULO04aig] C: MAIL FROM:
[2018-09-03 05:18:32] DEBUG [PFhdqz3yHs] S:
250 2.1.0 OK v72-v6sm36109733pfj.22 - gsmtp
[2018-09-03 05:18:32] DEBUG [PFhdqz3yHs] C: RCPT
TO:
[2018-09-03 05:18:32] DEBUG [0rULO04aig] S: 250 2.1.0 OK
v186-v6sm24113856pgd.25 - gsmtp
[2018-09-03 05:18:32] DEBUG [0rULO04aig] C: RCPT
TO:
[2018-09-03 05:18:32] DEBUG [PFhdqz3yHs] S: 250 2.1.5 OK
v72-v6sm36109733pfj.22 - gsmtp
[2018-09-03 05:18:32] DEBUG [PFhdqz3yHs] C: DATA
[2018-09-03 05:18:32] DEBUG [BlXN9zcX8Ak] S: 421 4.7.0 Temporary
System Problem. Try again later (10). x65-v6sm28504837pfk.140 - gsmtp
[2018-09-03 05:18:32] DEBUG [BlXN9zcX8Ak] Closing connection to the
server using "end"
[2018-09-03 05:18:32] INFO [#1] Connection #1 was closed
[2018-09-03 05:18:32] ERROR [#1] Pool Error for #1: Data command
failed: 421 4.7.0 Temporary System Problem. Try again later (10).
x65-v6sm28504837pfk.140 - gsmtp
[2018-09-03 05:18:32] ERROR Send Error: Data command failed: 421 4.7.0
Temporary System Problem. Try again later (10).
x65-v6sm28504837pfk.140 - gsmtp
[2018-09-03 05:18:32] DEBUG [gflzBwS1fc] S: 421 4.7.0 Temporary System
Problem. Try again later (10). b203-v6sm1382852pfb.174 - gsmtp
I am unable to post the complete code since the checker here keeps telling me that it is not properly formatted. The code follows ESLint rules.
Let me post a pseudocode:
forEach(users, key) =>
getSQL
create Users if no result found
sendEmail(user)
So the settings:
function Mailer(options) {
this.options = options;
that = this;
this.transporter = nodemailer.createTransport({
pool: true,
maxConnections: 50,
maxMessages: Infinity,
debug: true,
logger: true,
connectionTimeout: 3000,
host: config.mail.host,
port: config.mail.port,
secure: false,
auth: {
user: config.mail.username,
pass: config.mail.password,
},
});
}