I think this error is related to sending emails to a domain outside of the domain the SharePoint server lives in. Are you able to send emails using the built in “Alert me” functionality in the list view header?
Youa re correct. I found out alerts were not enabled in our 2016 farm and if alerts need to work for the email to work then this makes sense why it isnt working.. I was not trying to send them outside of our farm however. Thanks again!