nodemailer occasional ETIMEDOUT while alternate method is successful - node.js

Question

Context
Windows Server 2012 R2 Standard, not behind a network proxy
NodeJS 6.11.3
Nodemailer 4.1.0
This snippet tries to send email to an in-house SMTP server on the same network that does not require authentication:
var transporter = nodemailer.createTransport({
host: "smtp.example.com",
port: 25,
secure: false
});
var mailOptions = {
'from': 'noreply#example.com',
'to': 'someone#example.com',
'subject': 'test email',
'text': 'some text for a body',
};
transporter.sendMail(mailOptions, (error, info) => {
if (error) {
// do something
} else {
// do something else
}
});
The following is an alternate script (VBScript) that we're executing ad hoc just for testing.
Set objMail=CreateObject("CDO.Message")
objMail.From="noreply#example.com"
objMail.To="someone#example.com"
objMail.Subject="Test from new server"
objMail.TextBody = "This is a message."
objMail.Configuration.Fields.Item ("http://schemas.microsoft.com/cdo/configuration/sendusing")=2
objMail.Configuration.Fields.Item ("http://schemas.microsoft.com/cdo/configuration/smtpserver")="smtp.example.com"
objMail.Configuration.Fields.Item ("http://schemas.microsoft.com/cdo/configuration/smtpserverport")=25
objMail.Configuration.Fields.Update
objMail.Send
Set objMail=nothing
Problem
Set up a setInterval() to try to send email with Nodemailer once per minute.
Sometimes the email is sent but more often we get the following error.
{ Error: connect ETIMEDOUT XXX.XXX.XX.XX:25
at Object.exports._errnoException (util.js:1020:11)
at exports._exceptionWithHostPort (util.js:1043:20)
at TCPConnectWrap.afterConnect [as oncomplete] (net.js:1086:14)
code: 'ECONNECTION',
errno: 'ETIMEDOUT',
syscall: 'connect',
address: 'XXX.XXX.XX.XX',
port: 25,
command: 'CONN' }
No apparent correlation to restarting the server, frequency of calls to the script, etc.
What's most strange (to me) is that right in the middle of, say, 40 failed Nodemailer attempts, executing the VBScript sends an email through just fine. It never fails.
Resolution Attempts
I've looked at dozens of nodemailer usage examples and read the nodemailer documentation. I can't tell that I'm doing anything wrong; the firewall isn't blocking us, we're not behind a network proxy, the SMTP server doesn't require authentication.
I don't see any files named util.js or net.js that I could investigate.
I've read every Stack Overflow post mentioning the ETIMEDOUT error.
Questions
Am I doing anything wrong? If so, what?
If I'm not, then what could cause this behaviour? What should I ask my system / network admins to investigate?

Answer 1

This turned out to be caused by a problem outside my application. Thus, I'm answering the following portion of my original question set, in case someone else gets that same error message / experiences the same intermittent behaviour and is looking for possible causes outside her/his app: "[w]hat could cause this behaviour? What should I ask my system / network admins to investigate?"
In my example, the host parameter was set to "smtp.example.com" (which is, of course, not the real hostname). On our network, the hostname I was given can resolve to either of two IP addresses. When it resolved to one IP address, the SMTP server was reached and the message was accepted for queueing. When it resolved to the other IP address, the SMTP server was not reached and the timedout error was the result.
Until the network admins figure out the problem, I'm skipping the hostname and sending directly to the IP address that works.