I’m not the author of the plugin, but I just installed the plugin — and it doesn’t send emails by itself. It only allows you to configure your own custom SMTP server to send the mails.
So, as I wrote in your other post, there’s nothing to whitelist from the plugin’s perspective.
If you’re trying to allow the plugin to make an SMTP connection to your Exchange server, the IP address to whitelist would be the IP address of the server where the WordPress website is hosted. The plugin and its author have nothing to do with that ??
On the other hand, if something on the Webserver server is blocking an external 3rd-party SMTP server configured in the plugin from working, then it’s the IP addresses of the configured SMTP server that will have to be whitelisted — on the server’s firewall. Again, the plugin itself is neutral.