Workflow emails from Notification server are not being sent

When sending Workflow emails from Notfication server, no email is received.

In the Syracuse logs we can see the error

admin.mail | error | sendMailToSMTPServer Connection timeout Error: Connection timeout
at SMTPConnection._formatError (/Sage/Syracuse/syracuse/bin/@sage/x3-web-server/node_modules/nodemailer/lib/smtp-connection/index.js:790:19)
...

When I navigate to Administration, Administration, Servers, Notification Server
ACTIONS, Test configuration
In the Recipient email address put in an email addresses which is failing then click OK

This gives error "Cannot read properties of undefined (reading 'toString')"

I am also trying the commands from command line on the Syracuse server console

nmap -p 25 smtp-server.mydomain.com

This returns the following which looks normal

PORT STATE SERVICE
25/tcp filtered smtp

Running the command below hangs which seems to indicate a communication error

echo QUIT | telnet smtp-server.mydomain.com 25

As it seemed to be a communication error, on checking the SMTP relay it was found that some IP addresses were not whitelisted in the NSG rules

The Syracuse service did also need to be restarted before the emails were succesfully sent out.