Azure Function app cannot connect to on-premise SMTP server - azure

Question

I'm using Azure Functions to email an Excel file. I use the npm package NodeMailer to send the email. When I try to send the email I get this error in the Function app logs:
Result: Failure Exception: connect ETIMEDOUT [SMTP server IP address]:50025 Stack: Error: connect ETIMEDOUT [SMTP server IP address]:50025 at TCPConnectWrap.afterConnect [as oncomplete] (node:net:1161:16)
Here is part of the app code that sends the email:
// Send the email
const transporter = nodemailer.createTransport({
host: "[SMTP server IP address]",
port: 50025,
secure: false,
ignoreTLS: true,
connectionTimeout: 180000,
debug: true
});
let mailOptions = { .... }
context.log('Sending email');
let info = await transporter.sendMail({mailOptions, function(error, info) {
if (error) {
context.log("sent mail error: " +error.message);
}
context.log(info.response);
}
});
The 'Sending email' line prints out in the logs so I know the code gets that far. However, the code doesn't print out 'sent email error ...'.
I'm using an on-premises SMTP server that is hosted on a private network. I whitelisted the Azure Function app's outbound IP addresses so the app can access the SMTP server. The request from the Function app is not hitting the firewall on the private network(ie none of the outbound IPs are hitting the firewall).
I've tried tweaking the nodemailer configuration, like increasing the connection timeout time and enabling TLS with no difference.
I don't know what else to try. Is there any way to solve this?