“PostmanWpMail” Error, “Code_535″…?

So, I’ve been running my Contact Form 7 contact form on my Hostgator-hosted Site for a while now, no issues. Yesterday, I migrated the website to Siteground. Now my contact form is not working. I get the red-outlined box upon form submission, “We’re sorry! For some reason the message did not send…” I installed Easy WP SMTP, and the test emails from the app are going thru without a problem. A CS rep at Siteground cut-n-pasted this message logged from the Contact Form 7 submission: “ERROR PostmanWpMail: Exception code=535 message=Incorrect authentication data.”

Can anyone decipher what the problem is? What’s causing it? The Siteground rep told me to let them know if they need to set amy permissions differently. I would greatly appreciate some help with this ??

Thanks.

The page I need help with: [log in to see the link]