How to test where your mail is being sent from?

I am using the Subscribe by Email plugin and I have WP SMTP plugin setup to use our SMTP server to send all of our emails. We originally had it setup through SendGrid but had negative results as their IP addresses are listed on a bunch of spam block websites.

With that, I’m not sure our mail is being routed through our SMTP server and not sure where to check that it’s being done.

Secondly, with the plugin “Subscribe by Email” it’s knocking my server offline when I publish a post to “Subscribe by email” and it sends out to our thousands of subscribers. I’m not sure if this is because the server is trying to use PHP mail to send? But I’m also not sure how to check that.

Any help would be greatly appreciated. Also, how do I force it to use the WP SMTP plugin?